Learn Italian: Penso che l'attore vada sul palcoscenico adesso. - I think the actor is going on stage now.

The phrase penso che (I think that) expresses an opinion or uncertainty, which triggers the subjunctive mood in Italian. So, instead of the normal present tense va (he goes), we use the present subjunctive form vada.

Wait, andare is an -are verb. Shouldn't its subjunctive end in -i?

You are right that regular -are verbs end in -i for the singular subjunctive forms (like parli). However, andare (to go) is highly irregular. Its singular present subjunctive forms are all vada (for io, tu, and lui/lei). You just have to memorize this one!

Since vada is the same for io, tu, and lui/lei, how do we know who is going?

In this sentence, the subject is explicitly stated as l'attore (the actor), so there is no confusion. If you were just saying 'I think he is going,' you might add lui (penso che lui vada) to clarify, since vada alone could mean 'I go', 'you go', or 'he/she goes'.

Why do we use sul here instead of al or nel?