Learn Italian: L'attore ha scioccato tutta la città per essere arrivato in pigiama. - The actor shocked the whole city for having arrived in pajamas.

This is the past infinitive. We form it with the infinitive of the helping verb (essere or avere) plus the past participle (arrivato). Here, the preposition per introduces the reason or cause of the shock.

Why do we use essere here instead of avere for the past infinitive?

Just like in the passato prossimo (the regular past tense), verbs of motion like arrivare require essere as their helping verb. If the verb took avere, like mangiare (to eat), it would be per avere mangiato.

Why does arrivato end in "-o"? Does it change based on who is arriving?

Yes! Because the helping verb is essere, the past participle must agree with the subject performing the action. Since l'attore is masculine singular, it is arrivato. If it were an actress (l'attrice), it would be per essere arrivata.

Why does the Italian sentence use the singular in pigiama when the English translation says "in pajamas"?