Learn Portuguese (Portugal): Espero que ela tenha depositado a poupança. - I hope that she has deposited the savings.

The present subjunctive (deposite) would mean I hope she deposits it, referring to an action happening now or in the future. Using the perfect subjunctive (tenha depositado) shows that the action of depositing is already in the past. You are hoping right now about an action that has already been completed.

What triggers the subjunctive mood in this sentence?

The expression Espero que (I hope that). Verbs expressing wishes, hopes, or desires almost always force the following verb into the subjunctive mood because they describe something subjective rather than a declared fact.

Why doesn't the past participle depositado change to feminine to match ela or a poupança?

In Portuguese compound tenses formed with the auxiliary verb ter, the past participle never changes its ending. It stays in the default masculine singular form (depositado) regardless of the gender or number of the subject (ela) or the object (a poupança).

Is it common to use poupança in the singular to mean savings?

Yes. While English usually uses the plural word savings, Portuguese frequently uses the singular a poupança to refer to a person's saved money as a single collective amount, or to refer to a savings account. You will also hear the plural as poupanças, which means the exact same thing.