The future perfect tense is used to demonstrate a future event that has a definitive end date.
| Positive | Positive short form | Negative | Negative short form |
|---|---|---|---|
| I will have eaten | I'll have eaten | I will not have eaten | I won't have eaten |
| You will have finished | You'll have finished | You will not have finished | You won't have finished |
| She, he, it will have worked | She, he, it'll have worked | She, he, it will not have worked | She, he, it won't have worked |
| We will have started | We'll have started | We will not have started | We won't have started |
| They will have left | They'll have left | They will not have left | They won't have left |
The future perfect is used with a future time word, (and often with 'by') to talk about an action that will finish before a specific time in the future, but we're not sure exactly when.
The future perfect is used to indicate 'how long' an action will have lasted compared to another action.