Learn French: Tu dois étaler la pâte dans le moule. - You must spread the dough in the mold.

It can! La pâte is a very versatile word in French. Depending on the context, it can mean dough, batter, pastry, or pasta. Since we are talking about spreading it into a baking pan (le moule), it clearly means dough or pastry here.

Why is it le moule instead of la moule?

The gender completely changes the meaning of this word! Le moule (masculine) means a baking tin, pan, or mold. If you use la moule (feminine), you are talking about a mussel (the seafood).

Is there a difference between tu dois étaler and il faut étaler?

Both express obligation, but tu dois (you must) is specific to the person you are talking to. Il faut (it is necessary / one must) is impersonal and general. In a recipe, you will often see both, but tu dois makes it sound like someone is giving you direct, personal instructions.

Could I use an imperative command here, like étale la pâte?

Yes, definitely. Using the imperative étale (spread) makes it a direct command, which is very common in written recipes. Using tu dois (you must) softens it slightly into an instruction or piece of advice.