The future perfect is used for an event expected in the future, to be completed before another point in the future. For example: I will have finished my assignment by next Friday. On a time line it is: x (now) ____________ x (finished assignment) ____________ x (next Friday)

in English: "I will have returned" indicates that something will happen in the future. To say "I have returned" means you already did it. These would not be equivalent in English either. Perhaps you can use these in two full sentences as examples to help us better determine which to use when.