Marie ou toi devez y aller.

English Translation

Either Marie or you must go.

Since the sentence says toi, why is devez used? Is this correct?

Hi @corgwin24,

Yes, this sentence is correct.

In French, the subject is not just toi here, but “Marie ou toi” (so 2 people), which could be replaced by “vous”, 2nd person plural subject pronoun. That’s why devez (plural) is used here.