The Future Perfect is a tense used for actions that will be completed before a point in time in the future.
Example
-The film will have ended by the time Chester gets out of bed. At eight o'clock I will have left.
The future perfect tense is for talking about an action that will be completed between now and some point in the future.
Imagine that your friend wants you to come over and look after their dog for a few days while they are on a business trip. They want you to come over at midday, so they can show you where the dog food is, however you are busy this afternoon. So you ask your friend if you can come at 8pm instead.
“No, that won't work. At 8 o'clock, I will have left already.” She says.
The Formula
• Positive
- will have + (past participle)
“I will meet you at the restaurant for dinner, my class will have ended by 6pm.”
• Negative
- will not have + (past participle)
“We will not have eaten breakfast before we get to the airport tomorrow morning.”
• Question
- will +(subject) + have + past participle
“Will you have eaten lunch when we arrive?”