English already has will for a future action: I will finish at eight. The future perfect is for when that action is already done before a later time you are pointing at.
By eight I will have finished.
When you arrive, I will have left.
What the future perfect does
- You are talking about a time that is still ahead.
- Something is finished before that time.
- You are not saying the action happens at that time — you are saying it is already over by then.
Two times, one later
| Finished before (future perfect) | The later time |
|---|---|
| I will have finished | by eight |
| I will have left | when you arrive |
| She will have stretched | before the race |
The later time can be a clock, or another future event. The future perfect takes the one that is done first.