We construct the future perfect with a subject, 'will have' and the past participle of the verb. Whether the subject is singular or plural is not important, the formula remains the same.
subject + will have + past participle
She will have left by the time you arrive.
They will have eaten everything by the time you get there.
I will have finished by the time you can help.
We use the Future simple to talk about actions expected to be completed in the future. This could be at a specific time or before another event occurs.
She will have been training for a year before the tournament. (a year is the specific time in this example)
They will have been studying for 4 hours by midnight. (the specific time here is four hours)
He will have been working here for twelve years by the time he retires. (the process of working will be interrupted by retiring)
You can watch our video above for more advise on when and how to use the Future Perfect. Visit our write-up for more detailed information and a mind-map to help you remember the uses of Future Perfect.