The structure is [Subject + will have + past participle + object]
I shall have written my exercise by then.
He will have left before you go to see him.
I will have sent the assignment by Friday.
Tips: You’ll often need to know this tense for the FCE Use of English Part 4. Keep an eye out for time phrases using by.
Question: The match ____________ by the time we arrive.
Answer: will have (already) started
NOTE: The tense that comes after “by the time”, will help you determine what tense you need. Note the difference between these two sentences.
The match will have started by the time we arrive.
The match had already started by the time we arrived.