When one past action happens before another, English uses the past perfect. 'By the time we arrived, the movie ___ already started.' Which word fits?

The past perfect tense ("had" + past participle) marks the earlier of two past events, which is exactly why "had already started" is correct here — the movie starting happened before the group's arrival, and English needs this extra layer of tense to show that sequence clearly.

"Has" is present perfect, used for actions connected to now rather than sequencing two past events, and "did" is simple past, which would flatten the sentence into ambiguous timing without showing which action came first.

This distinction matters most in storytelling and reporting, where getting the sequence of past events wrong can genuinely confuse a listener about what happened before what — past perfect exists specifically to prevent that confusion.