“400 years.” The time of the sojourning of the children of Israel from the time of the weaning-feast of Isaac (Gen. 21:8-13) until the Exodus from Egypt and giving of the Law was 400 years. Israel was not enslaved in Egypt for 400 years, as most people believe. See commentary on Exodus 12:40.
“know, yes, know.” This is the figure of speech polyptoton, emphasizing that Abraham was to absolutely know this information. For more on polyptoton, and why it is translated the way it is, see commentary on Genesis 2:16.