During the Passover Seder we read that G‑d told Abraham that his descendents would be in Egypt for four hundred years. But from what I understand, the Israelites were actually in Egypt for little more than two hundred years. Surely G‑d did not error when He decreed four hundred years of exile. There must be a great explanation. Could you enlighten me?
Indeed, G‑d foretold four hundred years of exile. During the Covenant Between the Parts G‑d told Abraham: "You shall surely know that your seed will be strangers in a land that is not theirs, and they will enslave them and oppress them, for four hundred years." Nevertheless, a calculation of the years spent in Egypt reveals that they spent far less time there.
Our sages explain that the countdown of 400 years began with Isaac's birth. G‑d's promise does not refer to Egypt by name, rather to a "land that is not theirs." As soon as Abraham had a child, his seed were subjected to living in lands that were not theirs—including Canaan which wasn't "theirs" at the time.
Isaac was sixty years old when Jacob was born, and Jacob was 130 years old when he went down to Egypt. This means that 190 of the 400 years elapsed before the Israelites arrived in Egypt. So the Israelites were in Egypt for a total of 210 years.
Interestingly, when Jacob first instructed his sons to descend to Egypt, he said, "Go down ("רדו") there and buy [food]." The numerical value of the Hebrew letters of the word "רדו" ("redu," "go down") is exactly 210!