Rashi on Genesis 6:3/רש"י על בראשית ו׳:ג׳

1לא ידון רוחי באדם MY SPIRIT SHALL NOT STRIVE AGAINST MAN — My Spirit shall not be in a state of discontent and shall not strive with Myself because of man.

2לעולם ALWAYS — for a long time. Behold, My Spirit has been contending within Me whether to destroy or whether to show mercy: such contending (deliberation) shall not be forever — meaning, for a long time.

3בשגם הוא בשר FOR THAT HE ALSO IS FLESH — This is the same as בְּשָׁגַּם — that is to say: because this quality is also in him viz., that he is only flesh, and yet he does not humble himself before Me; what would he do if he were of fire or of some other resisting matter! Similarly we find שַׁ for שָׁ, (Judges 5:7) עד שַׁקמתי דבורה “Until I Deborah arose”, where it is the same as שָׁקמתי; so also (Judges 6:17) שַׁאתה מדבר עמי “that it is thou who talkest with me”, where it is the same as שָׁאתה — so also here.

4והיו ימיו וגו THEREFORE HIS DAYS SHALL BE etc. — For 120 years I will be long-suffering with them, and if they repent not I shall bring a flood upon them. If, now, you object, saying that from the birth of Japheth until the Flood there were only 100 years, remember that there is no “earlier” or “later” in the Torah (events are not always related in chronological order) (Pesachim 6b): the decree (regarding the Flood) was issued twenty years before Noah had any children — so we find in Seder Olam. There are many Midrashic explanations of the words לא ידון but this is transparently its plain sense.