Learn Italian: Credo che il regista vada dietro il sipario. - I believe the director is going behind the curtain.

Vada is the present subjunctive of the irregular verb andare (to go). We use the subjunctive here because the sentence starts with credo che (I believe that), which expresses an opinion.

Since vada is irregular, how do we know it means "he goes" and not "I go"?

In the present subjunctive, the first three singular forms (I, you, he/she) are identical. For andare, they are all vada. We rely on the subject, il regista, being explicitly mentioned in the sentence to know exactly who is going.

Why does regista take the masculine article il when it ends in "-a"?

Regista (director) is a noun that changes its article depending on the person's gender, but not its ending. It is il regista for a male director and la regista for a female director.

Can I say dietro al sipario instead of dietro il sipario?

Yes, you can. The preposition dietro (behind) can be followed directly by a definite article, like il, or it can be combined with the preposition a to make al. Both are perfectly correct and common in Italian.