The difference between the use of ‘when’ and ‘if’ appears to be difficult for Dutch students, because in Dutch these words can both mean ‘als’. In English they can be distinguished, because they are used in different situations but their grammatical context is similar.
If is used for conditions in which something may or may not happen in the future, depending on the circumstances. It can introduce a conditional clause:
If + present simple tense is followed by the main clause with ‘will’.
e.g. If I take a taxi I will catch the train
The main clause can also precede the if clause
e.g. It’s raining. We’ll get wet if we go out. (not: if we will go)
In the if clause the present tense is used to express the future
When is used for things which are relatively sure to happen:
e.g. When I get the information I will send it to you ( I know I will get the information)
When + present simple tense is followed by the main clause with ‘will’.
The main clause can also precede the when clause:
e.g. I’ll send you a postcard when I’m on holiday.
Conclusion: when and if are used with similar grammatical structures, but have a different meaning.