For the pattern
When [proposition], [result]
use present tense or present continuous tense in [proposition].
I'll try to explain, but not the jargon I use is not standard:
Propositions are statements of fact that can be true or false. These are a few examples of propositions:
- you send this letter
- you want to go home
- you are old
A when-clause is equivalent to the satisfying moments in time that satisfy the proposition. You could rewrite the when-clause as,
The moment [proposition] is true
Here's a full example:
When I play football, you will see me
Let's assume I'm going to play at 5:30pm. The sentence is equivalent to
At 5:30pm, you will see me
because 5:30pm is the satisfying moment.
As you can see, the present tenses are the only tenses that make sense here. Let's see what happens if you try to use the future tense:
*When I will play football, you will see me (this sounds awkward).
The issue is, if I already have plans, the statement "I will play football" is true right now. Therefore the present moment is a satisfying moment, so we could rewrite this as
Right now, you will see me
And that's clearly not what you mean to convey. Essentially "When I will..." is an unused construction. If you did want something like this, you could write "When I plan to play football, I'll give you a call".