How long were the Hebrews in Egypt? How long were they enslaved there?
The time from the arrival of Jacob and his family in Egypt until the Exodus was only 210 years. The 400 years in Gen.15:13 is from the birth of Isaac until the Exodus. Note that it doesn’t say there that Abraham’s seed would be “in Egypt” 400 years, but “in a land that is not theirs”, and this was true as soon as Isaac (Abraham’s seed; see Gen.21:12) was born. Isaac was 60 when Jacob was born (Gen.25:26), and Jacob came to Egypt when he was 130 (Gen.47:9). These 190 years must be subtracted from the 400, leaving 210 (Rashi on Gen.15:13).
In Ex. 12:40 the Torah refers to the Hebrews having been in Egypt for 430 years, but this cannot be meant literally, because Levi’s son Kehas was one of those who come to Egypt with Jacob (Gen.Ch.46), and the total lifespans of Kehas, Amram, and Moses don’t add up to that much (see Ex.Ch.6). The 430 years started even before Isaac was born (when Abraham was 100), and even before Abraham finally left Charan at the age of 75 (Gen. 12:4).
The Hebrews weren’t mistreated in Egypt until Joseph and his brothers had died. Joseph was 39 when Jacob came to Egypt, and Levi was about 42. Joseph lived to 110 and Levi to 137, so the mistreatment didn’t begin for the first 95 (out of 210) years. It was especially severe during the last 80 years, around the time of Moses’ birth, when it was decreed that the male children should be killed. The transition from no oppression to severe oppression happened over a 35 year period (95+35+80 =210).