When he saw that the cities were destroyed and the travelers ceased... This explanation raises the question: Why did he go to Gerar, which is very distant? Therefore Rashi brings the alternate explanation. Since he wished to distance himself from Lot, he traveled to Gerar which was an inhabited place. (Maharshal)

The angel prevented him... Rashi needed to explain this so the verses do not contradict each other. It is written later (v. 6): “I also prevented you from sinning against Me,” implying that Hashem prevented him, whereas this verse implies that Avimelech refrained on his own from coming near her. (Nachalas Yaakov)

To include her servants, camel-drivers, and mule-drivers... גמלים וחמרים is the correct text. It refers to the men who drive the camels and donkeys, [not the animals themselves].

It is true that at first you had no intention of sinning... Rashi is answering the question: Why is it not written, “I also know that you did this with an innocent heart and clean hands,” as Avimelech said about himself?

But, do not claim clean hands. You might ask: After all, was Avimelech in fact not free of sin, since he did not transgress? The answer is: Hashem was saying, “Not due to yourself are you free of sin, as you say. Rather, it is due to Me. You would have lain with her even though she told you she was his wife!”

And do not think: perhaps she has become repugnant in his eyes... Rashi explains this because “Return the man’s wife, for he is a prophet” implies Avimelech should return her only because Avraham is a prophet and Sarah is a married woman. But were he not a prophet, he would not return her although she is a married woman. How can this be? Illicit relations are forbidden to all Noach’s descendants! Therefore Rashi answers that “For he is a prophet” means: “And do not think...”

A plague that is unusual to befall any beings... I.e., “Deeds that ought not to be done” cannot refer to Avraham, as he had not performed any deed. He only spoke.

In order to make his words truthful, he responded in this fashion... I.e., Avraham should have said, “She is my brother’s daughter,” and then he would not have needed to say, “But not the daughter of my mother.” He said as he did to make truthful what he had said earlier: “She is my sister.” You might ask: In 12:19, [Avraham was asked a similar question.] Why did Avraham not respond to Pharaoh at all? The answer is: Pharaoh had asked him only once, so he did not respond. But Avimelech asked twice, therefore he found it necessary to respond. (Maharshal)

So that he be mollified and pray for him. [Rashi knows this] because for taking his wife, [gifts were not necessary]. As Avimelech did not touch her, it is like he never took her. [You might ask:] Why did Avimelech need to mollify Avraham, while Pharaoh did not? The answer is: Pharaoh was completely unaware [that he had taken a married woman], because Avraham on his own craftily told Sarah (12:13): “Please say that you are my sister.” But here, they asked Avraham if she is his wife or his sister. He feared they would kill him and was compelled to say, “She is my sister.” That is why Pharaoh was smitten only with a disease for which relations are harmful, and after he returned her he no longer needed a cure. Whereas even after Avimelech had returned her, mollified Avraham and gave gifts, nonetheless it says: “Adonoy restrained every womb.” This was because Avimelech was considered a nearly deliberate sinner, since they interrogated Avraham: “Is she your wife?” As it says in Bava Kama 92a, “When a guest arrives in a city... is he asked about his wife?” [see Rashi, v. 11].

Avimelech said it was out of respect for her, in order to mollify her... This means as follows: Avimelech really gave them to Avraham so that Avraham would pray for him, as Rashi explained on v. 14. If so, why did Avimelech say to Sarah that it was to honor her? [The answer is:] He said the opposite [of the truth] in order to mollify her. The Re’m asks: How does Rashi know that Avimelech gave them to Avraham in order to mollify him? Perhaps he really gave them in Sarah’s honor! The Re’m answers: Since Hashem said, “Now return the man’s wife... and he will pray for you,” we learn that Avimelech’s goal [in returning Sarah] was to mollify Avraham so he would pray for him. Assumedly, Avimelech gave the gifts for the same reason. But it seems to me [that the answer is:] If Avimelech really gave for her honor, why did he give to Avraham? According to what Avimelech said [his intentions were], he should have given them to Sarah, which would have honored her more! Perforce, the main [reason for] giving the gifts to Avraham was so Avraham would pray for him, and Avimelech said what he did to Sarah only to honor and mollify her.

According to Sarah’s words. על דבר cannot mean “because of,” as it does in Devarim 23:5 על דבר אשר לא קדמו אתכם, because [of the following difficulty:] it is written earlier (v. 6), “Also, I prevented you from sinning against Me.” This implies that Hashem deprived Avimelech alone of the ability to have relations. Yet in our verse it is written, “For Hashem had restrained every womb.” Perforce, in our verse על דבר means Adonoy did this “according to Sarah’s words.” And when it said (12:17) וינגע ה' את פרעה... על דבר שרי, on which Rashi commented: “According to her commands,” this explanation was learned from here, based on the phrases’ similarity. (author’s commentary)