Rashi on Deuteronomy 7:17/רש"י על דברים ז׳:י״ז

1כי תאמר בלבבך — You must admit that the word כי here denotes “perhaps": perhaps thou wilt say in thy heart, “Because they are many, I shall be unable to dispossess them". Do not speak thus, לא תירא מהם THOU SHALT NOT BE AFRAID OF THEM. — For it is not possible to explain it in one of the three other meanings of כי so that the words לא תירא מהם will then appropriately fit in with it.