Translation:When I have more time, I will study more.
Strictly speaking, it is "When I'll have more time, I'll study more". But the system says that's wrong.
Not sure about the sources for your "strictly speaking" assertion about "will" after "when".
You may want to have a look here:
We do not normally use will in clauses with if or with time words:
- I’ll come home when I
- We won’t be able to go out if it
- It will be nice to see Peter when he
will get homegets home.
- You must wait here until your father
Christine, you are correct that the future tense "will" in the "when clause" is literally what the Czech here is saying. But that is not how contemporary English says it!