What is future perfect tense?

Future perfect tense indicates an event that will be completed by a specific point in the future.

We will have finished the project by the end of the week.
The movie will have ended before we can get there.
She will have left her house by now.

The pattern for forming future perfect tense is as follows:

Will have + past participle

To form the negative, place not between will and have. To form a question, move will to the beginning of the sentence, in front of the subject.

They will have gone by now.
Will they have gone by now?
They will not have gone by now.