what is future perfect used for
The future perfect tense is quite an easy tense to understand and use. The future perfect tense talks about the past in the future.
eg: I will have turned 13 by next month.
The above sentence describes what will have happened next month.
Hope I am helpful to you,
To describe an action or an event in the future preceding another action or event in the future
Future Perfect Simple
I will have left for the airport by the time you come from the meeting.
They will have completed twenty-five years of their married life by the sixth of this month.