Rashi: The four hundred years of exile, which ended when Am Yisrael left Egypt, would begin with the birth of Yitzchak ¹ (as follows - 60 years until the birth of Yaakov, who was 130 when he went down to Egypt, plus the 210 (the Gematriya of the word "Redu") years that Yisrael lived in Egypt).

Rashi: HaSh-m is informing Avraham of the combined periods of exile and slavery, which add up to four hundred years. ¹ The former incorporate the 190 years that Yitzchak and Yaakov experienced exile in various locations, including Eretz Yisrael itself. ²

Ramban #1: Avraham needed to know that, although HaSh-m had promised him and his offspring Eretz Yisrael, before this would happen, they would be strangers... for four hundred years.

Seforno: He needed to know that, although HaSh-m had promised him and his offspring Eretz Yisrael, this would not happen before the sin of the "Emori" was complete ¹ and they deserved to be expelled from the land. He also informed him that the slavery and the subjugation would not go hand in hand, since the latter would not begin as long as his children were Tzadikim, only after they began to sin. ²

Ramban #2 (citing Ibn Ezra): He needed to know that his children would be strangers - incorporating slavery and subjugation, up to four hundred years, starting from now. ³

Maharal (Netzach Yisrael Ch. 1, p. 10): For an entire nation to be scattered in exile is a deviation from the natural order, a temporary aberration due to sin. The exile itself teaches that there will also be a Redemption. ¹

Maharal (Gevuros Hashem Ch. 60, p. 267): "Geirus" means to be subject to others in a foreign land. "Avdus" adds the element of forced servitude, as a slave to his master. "Inuy" teaches that they would enslave us to a degree greater than was necessary. ¹

Maharal (Gevuros Hashem Ch. 3, p. 27): Every nation possesses two aspects, their physical nationhood (Chomer) in which all nations are parallel, and the identity and mission particular to each individual nation (Tzurah). The Tzurah of Yisrael is that they are "the nation of the G-d of Avraham" (Tehilim 47:10). When we were in Egypt, in addition to being enslaved in the physical sense (Chomer), we were made to suffer. The Tzurah of Mitzrayim was diametrically opposed to that of Yisrael, due to our unique status as HaSh-m's nation, and the clash was evident. ¹

Ramban (in 12:10): It was a punishment; a. for leaving Eretz Cana'an after arriving there at the behest of Hashem, and b. for endangering Sarah by declaring that she was his sister. These two sins constituted a lack of faith; a. that Hashem would sustain him during the famine, and b. that He would save and protect him from those who would kill him for his wife. ¹

Nedarim 32a (#1) and Targum Yonasan: It was a punishment for querying Hashem concerning His promise to give him Eretz Yisrael. ²

Nedarim 32a (#2): It was for taking his students, Talmidei Chachamim, to fight in war.

Nedarim 32a (#3) and Da'as Zekenim: He prevented people from coming under the wings of the Shechinah by returning them to Melech S?dom. ³

Ramban: This was a punishment; a) for leaving Eretz Kena'an after arriving there at the behest of HaSh-m, and b) for endangering Sarah by declaring that she was his sister. These two sins constituted a lack of faith; a) that HaSh-m would sustain him during the famine, and b) that He would save and protect him from those who would kill him for his wife. ¹

Nedarim 32a (#1); Targum Yonasan: It was a punishment for querying HaSh-m concerning His promise to give him Eretz Yisrael. ²

Nedarim 32a (#2): It was for taking his students, Talmidei Chachamim, to fight in war.

Nedarim 32a (#3); Da'as Zekenim: He prevented people from coming under the wings of the Shechinah, through returning them to Melech Sedom. ³

Maharal #1 (Gevuros Hashem Ch. 9, p. 55): Perhaps all agree that we went down to Mitzrayim to learn Emunah in HaSh-m, by witnessing the great punishments that would befall the Egyptians and HaSh-m's great kindness to us. If so, there are three events which might display Avraham's shortcoming in Emunah (which needed to be rectified), as follows. a) In the war against the four kings, although one may not rely on miracles, one who truly believes in Hash-m is not afraid, and does not bring out Talmidei Chachamim from the Beis Midrash to where they ought not to be. b) Avraham asked for a sign that he would merit the tremendous gift of Eretz Yisrael [of which his descendants would need to remain worthy]; this nonetheless showed a slight lack of faith (refer to 15:6:0.2:1), therefore, they were exiled in order to attain complete Emunah. c) When Avraham refrained from accepting the people of Sedom, they were held back from acquiring Emunah in HaSh-m. We went down to Mitzrayim so that all the nations would hear of the great miracles of the Exodus, and [many would ] be inspired to convert. ¹

Maharal #2 (ibid. p. 56): The opinions differ as to why Avraham's descendants were punished specifically with slavery. a) A person's physical dimension (Chomer) is made to be subjugated and ruled over, and Talmidei Chachamim (who rule over their physicality) are those who ought to be truly free (Avos 6:2). So too, the Jewish people should be free from any foreign subjugation (Eruvin 54a). Avraham used Talmidei Chachamim, so too were the Jewish people subjugated in Mitzrayim. b) A slave is dependent on others, whereas a Ma'amin shows that he can stand on his own without dependency. Avraham was the first of the Ma'aminim, yet he asked for a sign, therefore so too, his descendants would be subjugated in exile. c) When Yisrael lends support to the nations, they receive the ability to overpower us.

Maharal #3 (ibid. p. 57): Avraham was the root of Am Yisrael, and a slight deficiency in the root will become very evident in the branches. What was the deficiency that Avraham exhibited? a) Avraham was a "Rosh," the first of the Avos, but he misused this trait to "rule" over and use his students. Therefore, the same happened to his descendants. b) Avraham was missing Midas ha'Din, the trait of Yitzchak. ² This led to his request for a sign, which brought Midas ha'Din upon his descendants. c) Avraham was missing Midas ha'Emes, the trait of Yaakov, who had no impropriety in his offspring. Avraham therefore declined to accept the people of Sedom as converts. So too, his sons would be enslaved, and [temporarily] given over from HaSh-m's possession to the Egyptians.

Maharal (Gevuros Hashem Ch. 9, p. 57): Avraham was the root of Am Yisrael, and a slight deficiency in the root will become very evident in the branches.

Maharal (Gevuros Hashem Ch. 9, p. 52): It was in any case fitting that we be enslaved, to publicize the Name of Hash-m in the world. However, this process would not be initiated if not for sin; therefore, even a slight sin was enough to set the plan in motion.

Maharal #1 (Gevuros Hashem Ch. 9, p. 53): Avraham himself was not exiled, so that Yishmael and Esav would not be included in the subsequent blessing of, "and afterwards they shall emerge with great wealth" (15:14).

Maharal #2 (ibid. p. 54): The entire world was created for Avraham's sake; it is impossible that he would become a stranger under others' control.

Maharal #3 (ibid.): Sometimes a father's personal merits protect himself from punishment, but are insufficient to protect his offspring.

Maharal (Derech Chayim 5:4, p. 225): Avraham, as the first of the Avos, was the indicator for the history of his descendants. Avraham was thrown into the fiery furnace, and HaSh-m brought him out, so too they would go down to Egypt and then be redeemed. Therefore, HaSh-m informed Avraham of this.

Maharal (Gevuros Hashem Ch. 10, p. 60): The opposition to Avraham always comes in the form of the number four. Four kings fought against him, and likewise the exile of his children lasted 400 years and spanned four generations. Four represents the Kochos ha'Chitzonim, represented by the four directions, and Avraham represents the center of the world, around whom the world's existence revolves. ¹