Learn Italian: I dinosauri sono esistiti molti milioni di anni fa. - The dinosaurs existed many millions of years ago.

In Italian, fa literally comes from the verb fare (to make or do), but when used with time expressions, it means "ago". Just like in English, you place it immediately after the amount of time: molti milioni di anni (many millions of years) + fa (ago).

Why does the sentence use sono esistiti instead of hanno esistito?

The verb esistere (to exist) is a verb of state or being. When forming the past tense, verbs that describe states, movement, or life changes take essere (to be) as their helper verb instead of avere (to have). Since i dinosauri is a "they" subject, we use the "they" form of to be: sono.

Why does the past participle esistiti end in an -i?

Whenever you use essere as your helper verb in the past tense, the action word (the past participle) has to match the subject in gender and number. The subject here, i dinosauri, is masculine plural. Therefore, the normal dictionary ending of esistito changes to the masculine plural ending -i.

Why do we need the word di in milioni di anni?