How does one use the future perfect tense and the future-in-the-past perfect tense?
We use the Future Perfect tense (will have done) to inform that something will happen before a certain moment in the future.
Let's suppose it is 9.00 and we are going to the cinema. The film starts at 9.15, but we are not in the cinema yet and we will not be there until 9.30.
We can say:
By the time we are at the cinema, the film will have started.
The sentence means that the film will start before a certain event in the future (i.e. before our arrival to the cinema)
Futiure Perfect in the Past (would have done) is used in the third conditional sentences.
If I had studied harder, I would have passed the exam.
The sentence makes an assumption on a past event which cannot be changed. It says, 'If I had studied' before the exam (which has already taken place), I would have passed the exam (the one which has already taken place and I have already failed it).