Rashi on Exodus 23:31/רש"י על שמות כ״ג:ל״א

1ושתי is of the root שית or שות. The ת is dageshed, because it comes in place of two ת’s (ושת-תי) — one is necessary because no grammatical form of שות can be without the ת of the root, and the other is a servile letter (part of the suffix).

2עד הנהר UNTO THE RIVER — the Euphrates.

3וגרשתמו means AND THOU SHALT DRIVE THEM OUT.