HaShem is considering whether to tell Avraham something He is about to do. But you may ask, why would HaShem consider this? Does not HaShem do as He pleases? He does, but at the same time, do not forget what you have learned. We are all partners with HaShem to build this world in the image that HaShem has envisioned. Avraham is a big part of this partnership, and his part is important. So, if you are of the mind that everything is preordained, sorry to prove you wrong. HaShem considers everything He does, and things were not planned to go according to the numbers from day one. If that is so, then why give us free will? If HaShem has already decided who will make it to the Kingdom to come, why go through this charade? No, and I have already explained this: HaShem wants us to partner with Him in this world, but not all of us. Only those who choose to be with Him and the ones who do not will not. Be fruitful, multiply, and increase in number; it is not so that we can procreate and fill the world. I have also explained this and what a remnant of the people means. HaShem knew that only a few people, of all the masses, would be born with the heart needed to be His people, and off of the billions of people that have died, still live, and will be born, only a few would make it. Why? Because this is not forced upon you. You get to choose what kind of person you will become.
HaShem always makes decisions to guide us and this world in the directions He wants it to go. It is not preordained, but it is a waiting time to see who will be the people who wish HaShem as their God and who can and want to take this world on the same path as HaShem envisions. To be a partner with Him for everlasting life.
Why is HaShem contemplating telling Avraham about the destruction of these nations? Because this is what HaShem is about to do, and HaShem has already decided to do it. This is why HaShem thought, “What I am about to do.” Once again, we are partners in finishing this creation. HaShem created this Earth for a specific goal, and you do not leave your partners out of the big decisions, just like when HaShem consulted with the messengers/angels and said, “We will create Adam in our image, in the image of God.” So, HaShem is trying to decide whether to give Avraham the honor of putting his input into the matter, just like He told the messengers/angels when creating Adam. But at the same time, HaShem has His own rules to follow, and these nations' sins are full, and they have sinned to the point where there is no coming back from it. HaShem’s law says that when your sin is full, that is it; you are done, and so these nations are done.

Ramban (1194-1270) commentary: AND THE ETERNAL SAID. I.e., to the host of heaven standing by Him, or to the angel messengers.
It is possible that the word amar (He said) refers to thought, meaning that He thought He should not keep it hidden from Abraham on account of these reasons. Similarly: I ‘said,’ in the noontide of my days I shall go; And he ‘said’ to slay David. Likewise all expressions of speaking within the heart refer to thought.

Or HaChaim (approximately 1696 - 1743) commentary: והשם אמר המכסה אני מאברהם את אשר אני עשה G'd had said: "Should I conceal from Abraham what I am about to do?" G'd chose the word המכסה deliberately. Abraham noted that the third angel had a mission which had so far not been revealed to him, in contrast with the missions of the other two angels which had become manifest to him already. One had come to heal him and the other to tell Sarah about her impending motherhood. Moreover, when the angels departed from him, Abraham noticed that two of them walked in the direction of Sodom. Abraham accompanied those two angels to see them off. He realised that these angels still had a mission to perform on earth, something which had been concealed from him. Abraham would become aware of those angels' mission after the event. G'd was concerned lest he would feel badly in retrospect if He would not now take him into His confidence. Our sages (Tanchuma item 5 on our portion) state that the reason G'd decided to take Abraham into His confidence was because Abraham entertained doubts about the justice of G'd having brought on the deluge. He found it impossible to believe that there had not been at least ten or twenty righteous people at that time. In his opinion G'd should have suspended judgment on their account. The words אשר אני עושה then mean: "as I am used to do," i.e. I do not destroy a nation unless there are no righteous people left.
G'd had another reason to reveal to Abraham what He had in mind for Sodom, seeing it had been Abraham who had saved the people of those cities at the risk of his own life when he rescued Lot. Inasmuch as Abraham thought that these people were safe, G'd thought He owed it to Abraham to inform him about His plans for them. He did not have to spell out what it was that He would conceal from Abraham as this is self-evident.

Radak (1160-1235) commentary: וה' אמר, the Lord, Who had appeared to Him initially to inform him of the sins of the Sodomites (13:13) informed Avraham now, after the departure of the angels, about His plans in detail. The Torah adds, in its narrative, what had prompted G’d to take this unusual step of informing Avraham of His plans, seeing that his own safety was not involved. G’d felt duty-bound to apprise Avraham of His intentions.

Rashbam (approximately 1085 – approximately 1158) commentary: המכסה אני מאברהם, the fact that I am about to overturn Sodom and its satellite towns. G’d reasoned seeing that Avraham was sure to tell his descendants of G’d’s promise to him to give the whole land of Canaan to his descendants, He would diminish these descendants’ inheritance by utterly destroying these towns and the valley which were part of area promised to the Jewish people as an inheritance. He therefore felt duty-bound to obtain Avraham’s consent of what He was about to do. We had learned in Genesis 10:19 that the boundaries of the Canaanite extended to regions that included the Jordan valley. G’d considered it inappropriate to deny Avraham fulfillment of part of His promise by making these parts of the land of Canaan totally useless to its inhabitants.

In this verse, HaShem explains why He decided to tell Avraham his intention, which was to destroy Sodom. HaShem said,

“For I do know him, who to the end, he will command his sons and his house, after him, and they will obey the path/way of YHVH, to do righteousness and justice.”

This means that HaShem knows Avraham and that He knows that Avraham will successfully teach his descendants in the ways of YHVH/HaShem to learn His Torah and keep all its laws to stay on the path of the true God of all in existence. Now think very hard. Who are the direct descendants of Avraham nowadays? The Jewish people. Now, HaShem said He knows that no matter what, the descendants of Avraham will follow the right path of HaShem because Avraham and his descendants have been doing so throughout all their generations, in ancient times and nowadays, just like HaShem said in this verse. So, in the words of the living God of all Creation, what faith do the Jewish people hold? Look it up for yourself. HaShem, our God, in His own words, tells us this is the right path. The Jewish people of faith, who follow God and keep His laws, are right in their beliefs. This is not what I think; it is what God says in His words, right in this verse.
Another thing we are being taught here is that it is a commandment to instruct our children in the ways of HaShem, our God. It is our responsibility to teach our children the ways of HaShem, in order for HaShem to be never forgotten and that false gods will not deceive them. To neglect this is to bring your children to destruction and ruin. Which in turn is your destruction and ruin. Your children, by not being properly instructed in the way of HaShem, are liable to not know about HaShem and are easy targets to follow other gods. If this happens, HaShem says that that person would be cut off and forgotten. Is this something you want for your children?

Now for the last part of the verse.

“To bring to pass, for Avraham, that which YHVH spoke of him.”

HaShem decided to tell Avraham His plans regarding the destruction of Sodom. This decision was to bring to pass what HaShem had promised Avraham. HaShem concluded that Avraham needs to know, and when he does, Avraham’s and his descendants' promises from HaShem will come true.

Rashi (approximately 1040 - approximately 1105) commentary: כי ידעתיו FOR I KNOW HIM — this is an expression denoting "affection", as (Ruth 2:1) "kinsman (מודע) of her husband," and (Ruth 3:2) “And is not Boaz our kinsman (מודעתנו), and (Exodus 30:17) “I have distinguished (ידעתיך) thee by name”. Still the primary meaning of these terms connected with the root ידע is really that of knowing, for whoever holds a person in affection attaches him to himself, so that he knows him well and is familiar with him. And why do I hold him dear?
למען הביא IN ORDER THAT [THE LORD] MAY BRING — Thus will he ever command his children saying “Keep the way of the Lord in order that the Lord may bring upon Abraham etc.” Since it does not say here ‘‘upon the house of Abraham”, but upon Abraham himself, we may infer that he who trains up a righteous son is as though he never dies (Genesis Rabbah 49:4).

Ramban (1194-1270) commentary: FOR I HAVE KNOWN HIM (‘YEDATIV’), TO THE END (‘LEMA’AN’) THAT HE MAY COMMAND HIS CHILDREN. Rashi comments: “For I have known him, as the Targum takes it, is an expression denoting affection, just as A kinsman (‘moda’) of her husband’s; And I know thee. Still the main connotation of all these expressions is that of knowing, for he who holds a person in affection and draws him to himself knows him well and is familiar with him. But if you explain it as the Targum does — i.e., “I know that he will command his children” — then the word lema’an (to the end) does not fit into the sense.”
It is possible that the word yedativ means “I have raised him and elevated him so that he shall command his children after him to do that which is right before Me, and therefore I will make him a great and mighty nation so that he should serve Me. In a similar sense are the verses: I know thee (‘yedaticha’) by name; What is man, that Thou knowest htm? Or the verse may be stating, I know that he will command, and in a similar sense is the verse, So that thine ox and thine ass may have rest (lema’an yanuach), meaning that he may have rest.
The correct interpretation appears to me to be that the word yedativ literally means “knowing.” He is thus alluding that G-d’s knowledge, which is synonymous with His Providence in the lower world, is to guard the species, and even the children of men are subject despite it to the circumstantial evil occurrences until the time of their visitation comes. But as regards His pious, He directs His Providence to know each one individually so that His watch constantly attaches to him, His knowledge and remembrance of him never departs, as it says: He withdraweth not His eyes from the righteous. There are many verses on this theme, as it is written, Behold, the eye of the Eternal is toward them that fear Him, and other verses besides.

Sforno (approximately 1470/1475-1549) commentary: כי ידעתיו/for/because I know him, as a symbol of righteousness.
למען אשר יצוה את בניו/to command his sons, G’d did all of the foregoing in order that Avraham would instruct his sons to emulate the ways of G’d having personal experience of G’d’s great love for mankind, seeing how His love extended even to the wicked.
ושמרו...לעשות צדקה ומשפט למען הביא ה' על אברהם את אשר דבר אליו. The ultimate purpose of G’d in revealing to Avraham His manner of meting out justice was to ensure that He would have an opportunity to fulfill all the promises He had made to Avraham. [promises which were largely conditional on Avraham’s offspring being true to his teachings. Ed.] (compare 17:7).

Chizkuni (13th Century) commentary: למען אשר יצוה את בניו ואת ביתו, “in order that he will command his sons and the members of his household, etc.” He should point out to them that unless they followed their father’s or master’s tradition, they might wind up just as the people of Sodom would in short order. [Note that here as well as at the time of the deluge, G-d was not punishing these people for their paganism but for not dealing fairly with one another. Ed.]

Rabbeinu Bahya (1255 – 1340) commentary: כי ידעתיו, “for I know him intimately,” The root ידע is used here in the sense of bestowing recognition on someone, just as in Psalms 144:3 מה אדם ותדעהו, “what is man that You should put him on a pedestal?” Alternatively, the meaning of the word is that “I have supervised him closely.” When G’d “knows” a person this implies that He keeps careful track of all that this person does and He protects him. The words כי ידעתיו single out Avraham from other ordinary human beings who do not enjoy the constant supervision by G’d of what they do and what happens to them. The righteous enjoy this advantage over their secular-oriented fellow human beings.
You should appreciate that G’d operates in this universe by means of both השגחה כללית and השגחת פרטית, “supervision of a general kind,” and “supervision of a particular, personal kind.” We have a verse in Jeremiah 32:19 which spells this out. The text is: גדול העצה ורב העלילה אשר עיניך פקוחות על כל דרכי בני האדם לתת לאיש כדרכיו וגו', “wondrous in purpose and mighty in deed, whose eyes observe all the ways of men, so as to repay every man according to his ways, and with the proper fruit of his deeds!” Considering other living creatures, however, G-d’s supervision of their fates is of a more general nature; it concerns itself only with preserving the respective species.
The השגחה פרטית, personalized supervision of mankind, operates in the following manner. 1) There is supervision in the sense that G’d is aware of all of man’s actions and thoughts; this extends both to Jews and to Gentiles alike. We know this from Psalms 33:15 היוצר יחד לבם, המבין אל כל מעשיהם, “He who fashions the hearts of them all, who discerns all their doings.” The supervision becomes manifest in G’d protecting people against mishaps. This type of benevolent supervision does not extend to Gentiles and not even to all Israelites except the righteous among them. G’d saves the righteous from experiencing the kinds of disasters which non-deserving people are prone to experience. This is meant by the words כי ידעתיו in our verse (according to the view of Nachmanides).
The words כי ידעתיו are words by G’d directed at man, i.e. G’d is informing our patriarchs that in order to qualify for this kind of benevolent supervision of their individual fates it is essential to be righteous. Once Avraham was aware of this he would have powerful incentive to command his offspring to observe G’d’s commandments. This is spelled out in the words: “so that G’d may indeed bring upon Avraham all that He has promised him concerning them.” The reference once again to the four-lettered Ineffable Name י-ה-ו-ה is an allusion to the same attribute of G’d with which the patriarchs had not become familiar as guiding their own fates, as G’d dealt with them on the basis of the attribute Shaddai as we have explained already in connection with 17:1.
למען אשר יצוה את בניו ואת ביתו אחריו, “in order that he may command his sons and household after him, etc.” Rabbeinu Chananel writes as follows concerning this verse: “The “mere” words of the patriarchs in their lifetime were as binding as what is recorded in the written Torah at the time of the revelation.” This teaches you that oral traditions handed down from the times of the patriarchs are as binding as laws written down in the text of the Torah as direct commandments from G’d. After all, only the Ten Commandments were heard by the people directly from the mouth of G’d. The remainder of the written Torah was communicated to us by a prophet, i.e. Moses, who wrote them down and taught them to the people. Just as there is no qualitative difference between the Ten Commandments and the other commandments in the Torah, so there is no qualitative difference between traditions taught by Avraham, Yitzchak and Yaakov (who were all prophets) and by Moses.” Thus far Rabbeinu Chananel. [Clearly these words were aimed specifically at the Karaites.]

The commentary by Rabbeinu Bahya is like going around in circles, as this law of teaching your children of HaShem and His way is written in the Torah, and before Moses wrote the Torah, it was all oral. Case in point the next commentary by Rabbeinu Chananel.

Rabbeinu Chananel (approximately 980–1055) commentary: למען אשר יצוה את בניו ואת ביתו אחריו, the dictates by the patriarchs to their respective children, all only by word of mouth, without there being a written document claiming to be G’d’s instructions such as the Torah nowadays, were observed by these children of the patriarchs as if the Torah had already been given, and as if they each had said: “I will do and I will hear.” This is further proof that the oral Torah is not subordinate to the written Torah. All of what was written in the written Torah was included in what the people heard at Mount Sinai, as well as all that G’d taught Moses while he was on that mountain. Just as there is no difference between the specific commandment the people had heard, i.e. the Ten Commandments, and the remainder of the written Torah, so there is no difference [in the authenticity and binding nature Ed.] of the commandments not spelled out in the written Torah, but handed down by the sages orally from generation to generation. (Rabbeinu Bachya)

Radak (1160-1235) commentary: את ביתו ואת בניו, both the members of his household who were not biologically related to his as well as his family, in order to teach us that a man is obligated to monitor the conduct of all members of his household and is responsible for their misconduct if he did not use his authority to put a stop to it. David spells out this responsibility of the head of the household in Psalms 101:6-7 when he said: עיני בנאמני ארץ לשבת עמדי הולך בדרך תמים הוא ישרתני. לא ישב בקרב ביתי עושה רמיה, “my eyes are on the trusty men of the land, to have them at my side. He who follows the way of the blameless shall be in my service.” He who deals deceitfully shall not live in my house;” Concerning the wicked Solomon says in Proverbs 29:12: “a ruler who listens to lies, all his servants become wicked.”