It may be too much to start anuvver syntactical fred right after Ullrich's, but at this moment when I can hardly see over the piles of exam scripts lying on my desk, the question arises. Do you see a difference between the following two sentences?

A) "When I finish, I'll return the key."
B) "When I have finished, I'll return the key."

For me, these two are interchangeable, but some of my students claim that in A the person speaking has not yet started the activity, though they'll begin right away, whereas in B they have already started the activity. I think both sentences could be used in either of these two cases, with activity started or not.

What do you think?

It's the same thang as tsuwm asked about.

Common parlance: interchangeable.

Students: nix. As follows:

One literally says that they'll return the key at the same time that they finish, the other, that they'll return it after they've finished.

Edit: Anyone who says "after they've" begs the question will be shot.