Learn Italian: Se solo lui corrispondesse i miei sentimenti!

The phrase se solo (if only) introduces an optative sentence, which expresses a strong standalone wish. To express a wish about the present or future that feels unlikely or impossible, you must use the imperfect subjunctive. Using the present subjunctive with se solo is grammatically incorrect in Italian.

Doesn't corrispondere usually take the preposition a, like in "corrispondere a qualcosa"? Why is there no a here?

The verb corrispondere changes meaning based on whether it is followed by a preposition. When it takes a (corrispondere a), it means "to match" or "to correspond to". When used directly with an object without a preposition, as it is here, it means "to reciprocate" or "to return" a feeling or action.

Since the verb form corrispondesse already makes it clear that the subject is third-person singular, why is lui included?

While Italian usually drops subject pronouns when the verb ending makes the subject clear, adding lui adds emotional weight and emphasis. It stresses "If only he (specifically him, as opposed to someone else) reciprocated my feelings."

Why is there no main clause (a "then" part) following this "if" sentence?