Learn French: L'ascenseur est descendu très vite. - The elevator went down very fast.

In the passé composé (past tense), most verbs use avoir (to have) as their helper verb. However, a specific group of verbs that describe motion or a change of state use être (to be) instead. Because descendre (to go down) is one of these motion verbs, we must use est.

How is the word descendu formed?

It is the past participle of the verb descendre (to go down). For regular French verbs ending in -re, you drop the -re and add -u to make the past participle. So, descendre becomes descendu.

If past tense verbs with être have to match the subject, why doesn't descendu have an extra ending?

You're right that when using être as a helper verb, the past participle must match the gender (masculine/feminine) and number (singular/plural) of the subject. Here, the subject is l'ascenseur (the elevator), which is masculine and singular. Therefore, descendu stays in its default masculine singular form, so no extra -e or -s is added.

Could I say très rapide instead of très vite?