In English, there are sometimes exchanges like the following:
- Person A: If Bob finishes his thesis, ...
- Person B: When Bob finishes his thesis ...
The effect of B's response to A, and their use of the word when, indicates that Bob will finish their thesis and that this is not in question.
Similarly, in the other direction, we get exchanges like the following:
- Person A: When Bob finishes their thesis, ...
- Person B: If Bob finishes their theses, ...
Here, B's use of if calls into question the idea that Bob will definitely finish their thesis.
I assume that it's possible to make the same kind of distinctions in German, but how do speakers do this? My school-learned German has the same word, wenn used for the English senses of both if and when.