Re: 'must/have to' and 'going to/will'
Welcome to the forums, Petya.
Your usage was fine. To use "must" instead of "have to" is also fine. Not better. Not the only correct way. Just "equally good."
To be honest, more natural would be "Check the correct answer" and "Use these words to make a sentence" instead of either "you have to..." or "you must..."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.