Learn Italian: Dovete aiutare la nonna. - You all have to help grandma.

When you have a modal verb like dovere (to have to / must), only the first verb is conjugated. The second verb stays in its dictionary form (the infinitive). So, it is dovete aiutare (you all have to help), not dovete aiutate.

Who is doing the action in dovete?

Dovete is the "you all" (plural) form of the verb dovere. You use this when speaking to two or more people. If you were telling just one person they have to help, you would use devi.

Can dovete mean both "must" and "have to"?

Yes! Italian uses the same verb, dovere, for both. You can translate dovete aiutare as "you all must help" or "you all have to help"—the meaning is exactly the same in Italian.

Why does it say la nonna (the grandma) instead of using a word for "your"?

In Italian, if it is obvious from the context whose family member it is, speakers often just use the definite article la (the) instead of a possessive like "your". Notice that because we aren't using a possessive word here, we need the article. If you wanted to explicitly say "your (plural) grandma", you would drop the article and say vostra nonna.