Learn Italian: Se solo smettesse di illudersi invano! - If only she stopped deluding herself in vain!

The phrase se solo (if only) introduces an optative subjunctive, which expresses a strong wish. To express a wish for the present or future, you must use the imperfect subjunctive. This is very similar to how English uses a past tense for present wishes (If only she stopped).

Since smettesse can refer to 'I', 'you', 'he', or 'she', how do we know who the subject is?

The imperfect subjunctive endings for io, tu, and lui/lei are identical for this verb. We rely entirely on context to know it means 'she' stopped. If the context were ambiguous in real life, you would simply include the subject pronoun: Se solo lei smettesse...

Why do we need the word di before illudersi?

The verb smettere (to stop or quit) requires the preposition di when it is followed by another verb in the infinitive. To say 'to stop doing something' in Italian, the pattern is always smettere di fare qualcosa.

How is the reflexive pronoun si attached to illudersi?

When an infinitive verb is reflexive or takes an attached pronoun, you drop the final -e of the infinitive and attach the pronoun directly to the end. Here, illudere (to delude) drops the -e and adds si to match the implied third-person subject.