Learn Italian: Perché hai dovuto rimandare la riunione? - Why did you have to postpone the meeting?

In the passato prossimo, modal verbs like dovere (to have to) borrow the helper verb of the infinitive that follows them. Because rimandare (to postpone) takes avere, the modal verb also takes avere, giving us hai dovuto.

If the following verb took essere, would hai dovuto change?

Yes! If you change the infinitive to a verb that takes essere, like uscire (to go out), the helper verb changes too. For example, 'You had to go out' would be sei dovuto uscire (or sei dovuta uscire if speaking to a woman).

Why doesn't the past participle dovuto change to match the feminine word la riunione?

When you use avere as your helper verb, the past participle normally stays in its default -o form. It doesn't change to match the subject or the object following it. If the sentence required essere instead, then the ending of dovuto would change to match the subject.

Why is riunione feminine (la riunione) even though it ends in -e?