Future Perfect has two different forms: “will have done” and “be going to have done.” Unlike Simple Future forms, Future Perfect forms are usually interchangeable.
USE 1- Completed Action Before Something in the Future
The Future Perfect expresses the idea that something will occur before another action in the future. It can also show that something will happen before a specific time in the future.
Notice in the examples above that the reference points (marked in italics) are in Simple Present rather than Simple Future. This is because the interruptions are in time clauses, and you cannot use future tenses in time clauses.
USE 2- Duration Before Something in the Future (Non-Continuous Verbs)
Although the above use of Future Perfect is normally limited to Non-Continuous Verbs and non-continuous uses of Mixed Verbs, the words “live,” “work,” “teach,” and “study” are sometimes used in this way even though they are NOT Non-Continuous Verbs.
REMEMBER- No Future in Time Clauses
Like all future forms, the Future Perfect cannot be used in clauses beginning with time expressions such as: when, while, before, after, by the time, as soon as, if, unless, etc. Instead of Future Perfect, Present Perfect is used.
The examples below show the placement for grammar adverbs such as: always, only, never, ever, still, just, etc.
ACTIVE / PASSIVE
Check out some of the other ‘Verb Tenses’