Perfect Tenses and the Order of Events

Sentences that describe two events need a way to show which one happened first. Simple past tense just says something happened: the train left. It doesn't say when, relative to anything else. Past perfect (had left) does that extra job: it marks an event as already finished before another past event started. That's why 'By the time we arrived, the band left' sounds scrambled. Both verbs sit in plain past tense, so the sentence just lists two things that happened, with no signal about order. Fix it to 'had left' and the timeline snaps into place: the band's departure finished before the arrival even began.

The same logic runs forward in time. Future perfect (will have finished) marks an event as complete before some future moment. 'By June she will have finished the draft' sets June as the checkpoint and places the finishing before it. Drop the 'have' and 'will finish' just predicts an event with no checkpoint attached. The signal word is usually a phrase like 'by the time,' 'by,' 'already,' or 'before,' which flags that one event needs to be anchored earlier than another.

This sits next to every other tense-and-sequence question in usage conventions: once you accept that a sentence is a built timeline, choosing had left over left, or will have finished over will finish, is the same move as choosing a verb tense that matches a nearby clause. Look for the earlier event, give it the perfect form, and let the other verb stay in its own simple tense.