Future Perfect tense

Question

I cannot understand the usage of this tense. For example, what does "I will have done this task" mean? Does it mean the effects of this task will be remaining in the future?

On a more general note, please tell me in which situations I can use this tense.

Answer 1

I will have done this task when you arrive

means I am expressing my certainty that by the time you come, the task will be completed. This construction is used to show that something will be finished when something else happens in the future.

Answer 2

I will have done this task when you arrive

This is not a very natural statement in English. You would probably say:

I will have done this task by the time you arrive

We can break this down into two things that happen:

1. The task is completed
2. You arrive

What the statement is saying is that 1. will happen before 2. happens. I will complete the task, and then you will arrive. This is usually the meaning of the future perfect:

They will have eaten their dinner when we get there

1. They eat dinner
2. We get there

They will have painted the house in time for Christmas

1. They paint the house
2. Christmas time comes

In these constructions, item 1. is always completed and finished before item 2. happens.