Learn Italian: Dopo essere sceso dalla giostra avevo le vertigini, quindi mi sono seduto. - After getting off the ride I felt dizzy, so I sat down.

The past infinitive uses the exact same helping verb that the main verb takes in the passato prossimo. Because scendere (to go down or get off) is a verb of motion, it requires essere instead of avere.

Does the ending of sceso change depending on who is speaking?

Yes! Whenever a compound tense uses essere as the helping verb, the past participle must agree with the gender and number of the subject. Here, the speaker is male. If a woman were speaking, she would say dopo essere scesa, and later in the sentence, mi sono seduta.

Why does this sentence use both the imperfect tense (avevo) and the passato prossimo (mi sono seduto)?

The imperfect tense (avevo le vertigini) describes an ongoing physical state in the past (feeling dizzy). The passato prossimo (mi sono seduto) is used for the specific, completed action that happened as a result of that state (the sudden act of sitting down).

I see avevo means 'I had', but the English translation says 'I felt'. How does avevo le vertigini work?

Just like being hungry (avere fame) or being cold (avere freddo), Italians express dizziness using the verb avere (to have). Avere le vertigini literally translates to 'to have the vertigos', which is the standard way to say 'to feel dizzy' in Italian.