בראשית כט:ח פירוש: רש״י וSiftei Chakhamim

לא נוכל WE CANNOT — give them water because the stone is a heavy one.

To give them drink because the stone is large. Rashi is explaining that they cannot roll it due to its weight, as Rashi said on v. 3, and not because they had so agreed among themselves.

וגללו AND THEY ROLL AWAY — This word is rendered in the Targum by ויגנדרון “and they will roll away” (whilst in Genesis 5:3 it is rendered by an Aramaic participle) because the verb has here a future meaning.