Learn Italian: Se avessi un castello immenso, che cosa sognaresti di fare? - If you had an immense castle, what would you dream of doing?

This sentence is a second conditional, which talks about an unlikely or hypothetical situation. In Italian, the se (if) clause for a hypothetical situation must use the imperfect subjunctive, which is avessi here. You cannot use the regular past tense for hypothetical 'if' clauses.

Since avessi is both the 'I' and 'you' form in the imperfect subjunctive, how do we know who is the subject here?

We know it is 'you' (tu) because of the main verb in the sentence. sognaresti is strictly the 'you' form of the present conditional. The context tells us the subjects match, so avessi refers to 'you' as well.

Why do we need the word di before fare?

In Italian, when the verb sognare (to dream) is followed directly by an action in the infinitive (like fare), it must be connected with the preposition di. So, 'to dream of doing' is always sognare di fare.

Can I just say cosa sognaresti instead of che cosa sognaresti?

Yes! When asking 'what' as a question word in Italian, you can use che, cosa, or che cosa. They all mean the exact same thing in this context. Che cosa is just a slightly more traditional way to say it.