Study this example:

Will you come over to my place tomorrow?

I’ll come when I finish work.

 

“I will come when I finish work.” is a sentence with two-parts: the main part “I will come” and the when – part “when I finish work.”

The time in the sentence is future (tomorrow) but we use a present tense in the when-part of the sentence.

The same rule applies after while / before / after / as soon as / until / till.

 

We can also use the Present Perfect after when / after / until / as soon as.

eg. Can I use your pen when you’ve finished with it?

 

If we use the Present Perfect one action must be finished before the other so they don’t happen together.

eg. When I’ve finished cooking, we can watch TV. (First I’ll cook and then we can watch TV.)

 

Do not use the Present perfect if the two things happen together. Use the Present Simple Tense.

eg. When I see James, I’ll ask him about the football match. (not When I’ve seen James)