Genèse 15:13

And he said to Abram, Knowing, thou must know, that thy sons shall dwell in a land not their own, because thou hast not believed, and they will subjugate and afflict them four hundred years;

THAT THY SEED SHALL BE A STRANGER. This is a verse that is to be transposed, its purport being that “thy seed shall be a stranger for four hundred years in a land that is not theirs, and they shall enslave them, and they shall afflict them.” He has thus not specified the length of the period of servitude and affliction.
There are many cases in Scripture where verses must be transposed if they are to be interpreted properly. Thus: There came unto me the Hebrew servant, who thou hast brought unto us, to mock me; And all countries came into Egypt to buy corn to Joseph;, 41:57. The meaning being: “And all countries came into Egypt to Joseph.…” For whosoever eateth leavened bread, that soul shall be cut off from Israel, from the first day until the seventh day; In that day a man shall cast away his idols of silver, and his idols of gold, which they made for themselves to worship, to the moles and to the bats; Come, and hearken, and I will declare, all ye that fear G-d, what He hath done for my soul; They cry unto Me, My G-d we Israel know Thee; And they shall be Mine, saith the Eternal of hosts, in the day that I do make, even Mine own treasure, and I will spare them. There are many other such verses.
The sense of the verse is: “Even though I tell you that I have given this land to your children, you should surely know that before I give it to them they shall be strangers for four hundred years in a land not belonging to them, and they shall also enslave them and afflict them.”
Rabbi Abraham ibn Ezra said [that the verse should be interpreted as follows]: “You should surely know that your children shall be strangers in servitude and affliction until the end of a four hundred year period commencing from this day of the covenant.” If so, G-d informed Abraham of the time of the redemption, but He did not inform him of the exact length of the exile. This too is correct.

ידוע תדע, here G'd informed Avram why it would take such a long time before His promise to Avraham that his descendants would inherit the land of Canaan could be fulfilled. G'd would not be justified in expelling a nation from its homeland until its measure of sin is full to overflowing. Therefore G'd told Avram ידוע תדע כי גר יהיה זרעך בארץ לא להם, that his own descendants would be strangers in a land which is not theirs until such time as the local inhabitants could legally be expelled by G'd from the land of Canaan. At the same time, He also told him about what would happen to his descendants in the interval, i.e. enslavement, cruel oppression, etc., for at least some of the intervening years. However, G'd implied that such suffering would not occur during the periods when Avram's descendants would be righteous. We know from the dates when the sons of Yaakov died, that as long as even a single one of them survived, no restrictions of any kind were imposed on the Israelites in Egypt, in spite of their having increased at a phenomenal rate already. All of these developments are attested to in Ezekiel in 20, 8-9 when he referred to the descendants of Yaakov in Egypt becoming rebellious and refusing to honour their tribal obligations vis-à-vis the G'd of Avraham-Yitzchok, and Yaakov. The reason why the prophet dragged up such ancient history was only in order to make plain to the newly exiled nation that their misfortunes had always been due only to their betrayal of their covenant with G'd. Compare Isaiah 48,5 ואגיד לך מאז בטרם תבא השמעתיך פן תאמר עצבי עשם וגו' "therefore I told you long beforehand, announced things to you ere they happened; that you might not say: 'my idol cause them, my carved and molten images ordained them."

ויאמר...בארץ לא להם, which will never be theirs. G’d describes the lands in which the Israelites will be exiled as countries which were not included among those of which G’d had said that Avram’s descendants would inherit them. They would remain strangers in a country or countries not theirs until the conclusion of the period of 400 years of which G’d speaks here. G’d speaks of 400 years and an additional 30 years commencing now that G’d revealed this to him. [his descendants could not begin to be exiles until Yitzchok was born, 30 years after the date of this vision. Ed.] This prophecy was fulfilled, the Jewish people leaving Egypt at the conclusion of 400 years after the birth of Yitzchok, or 430 years from the date of these revelations. On the face of it, in light of Avram having been 75 years old when he migrated from Charan, we can only account for 425 years from that time until the redemption of the Israelites from Egypt. We need to assume therefore that the vision involving the 400 years was experienced by Avram while in Charan, five years before he moved to Canaan. At any rate, G’d did not elaborate how many of these 400 years would be spent in Egypt, or even if any would be spent in Egypt. Our sages however, (Seder Olam chapter 3) have said that 210 years of the 400 are accounted for by the stay in Egypt, basing themselves on an allusion contained in the numerical value of the word רדו being 210. (compare Genesis 42,2 רדו שמה). The details of the 400 years are as follows: we start with Yizchok’s birth. He was 60 years at the time Yaakov was born, and Yaakov, by his own account was 130 years old when he answered Pharaoh’s question as to his age. We therefore have the first 190 years accounted for as predating the descent of the family to Egypt. It is totally impossible to believe that the 400 years referred to by G’d’s announcement to Avram were all spent in Egypt, seeing that Kehot, a grandson of Yaakov was among those who moved to Egypt with his family, and his age at death is given by the Torah as 133 years. We know that Moses was 80 years at the time when he made his first appearance before Pharaoh, about a year before the Exodus. If the Israelites had stayed in Egypt for 400 years, then Amram, Moses‘ father, even if he had been born during the last year of the life of Kehat would have lived for 187 years assuming that Moses was born during the last year of Amram’s life. Both of these assumptions are highly unlikely, as is the likelihood of Amram living more that 187 years without the Torah mentioning any of this. In fact, the Torah specifically states that Amram lived to the age of 137 (Exodus 6,20) It is most likely therefore, that the count of 400 years does indeed begin with the birth of Yitzchok. As long as Avram’s descendants were so few in number, they most certainly deserved to be described as “strangers” even in the land of Canaan, seeing they did not own even a small part of it. If you add the fact that Yitzchok spent many years in the land of the Philistines, and Yaakov spent 20 years with Lavan in Aram, a land which would never be part of Eretz Yisrael, neither of them could truly be perceived as a resident of the land of Canaan.

ועבדום, וענו אותם, “they will serve them; and they (the masters) will oppress them.” This was the answer to Avraham’s question: במה אדע כי אירשנה, “how will I know that I will indeed inherit it (the land) even vicariously,” in verse 8, where Avraham asked for some kind of guarantee that G–d’s promise to him would not be negated by his descendants forfeiting their claim to its fulfillment.

ארבע מאות שנה, “for four hundred years.” This is a separate revelation from G–d to Avraham. Another interesting explanation is that the numerical value of the letters in Avraham’s question במה adds up to 400 when you use the system known as א-ת, ב-ש, ג-ר, i.e. substituting the letters commencing by reading the alphah-bet backwards. Furthermore, the Talmud in tractate Nedarim folio 32, comes to the conclusion that the Jewish people had to suffer the penalty for their founding father having asked G–d for proof that His promise would come true, [although seeing that he had been unable to have children until that time would seem to us as a reasonable question. Ed.] Another reason given is that when Avraham agreed to the King of Sodom to have his citizens whom he had taken prisoner restored to him, he is punished for missing out on a chance to convert these people to monotheism. (Compare Genesis 14,21)