Hi Susan, the future perfect is used for describing a future (hypothetical) event that is in the past relative to another future event. So both events are in the future, but one is in the past with respect to the other.
For example, "There's a detour, so I can't stop by John's tonight, because I will have driven home a different way." or "By the time I wake up I'll have only had 4 hours sleep!"
It's quite common to have to use it. I hope that helps!
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