The Future Perfect is used to show the time by which something finishes. For example:
- I will have finished by 4.00. - By 4.00 I will finish a certain activity.

The Future Continuous is made with will + have + the past participle. For example:
- I will have arrived by 4.00.
- They will have left by next week.
- He will have gone to America by the end of the month.

Negatives add not, for example:
- I won't have arrived by 4.00.
- They won't have left by next week.
- He won't have gone to America until next month.

Questions invert will and the subject, for example:
- Will you have arrived by 4.00?
- Will they have left by next week?
- Will he have gone to America by next month?

The Future Perfect shows the time before which something will finish. For example:
- Next Friday, I'll have worked here for 5 years.
When next Friday comes, it will be 5 years since I started working here.

We often use by to show the time before which something is completed, for example:
- Can I borrow you book? - You can have it tomorrow, I'll have read it by then.
- I'll have left by the time you arrive.

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