Learn Italian: Ci siamo divertiti molto alla festa. - We had a lot of fun at the party.

The base verb is divertirsi (to have fun), which is a reflexive verb. In the past tense, all reflexive verbs must use essere (to be) as their helping verb. That is why we use siamo (we are) instead of abbiamo (we have).

Why does divertiti end in -i?

Whenever a past tense uses essere as its helping verb, the past participle (here, divertiti) has to match the subject in gender and number. Since the subject is "we" (a plural group), it takes the plural ending -i. If the group were made up entirely of women, it would be divertite.

What is Ci doing at the beginning of the sentence?

Ci is the reflexive pronoun meaning "ourselves". Because the dictionary form of the verb is divertirsi (literally "to amuse oneself"), we need ci to match the "we" subject. In the past tense, this pronoun always goes right before the helping verb (siamo).

Why doesn't molto change its ending to molti to match the plural subject?