Learn Italian: Hai chiuso la porta? - Did you close the door?

It is the irregular past participle of the verb chiudere (to close). Instead of following the normal rule and ending in -uto, it changes to chiuso. Because this is a high-frequency verb, you just have to memorize this special form!

Why is there no Italian word for "did" in this question?

Italian doesn't use helper words like "do" or "did" to ask questions. Instead, you just use the standard past tense—in this case, hai chiuso (literally "you have closed")—and raise your voice at the end of the sentence to show it is a question.

Since la porta is feminine, shouldn't chiuso change to chiusa to match it?

No. When you form the past tense using the helper verb avere (like hai here), the past participle usually stays in its default -o form. It does not change its ending to match the object (la porta).

How do we know it means "Did you close" if the word tu (you) isn't in the sentence?

The helper verb hai is the "you" form of avere. Because the verb itself tells us exactly who is doing the action, Italian speakers usually drop the subject pronoun (tu) entirely.