A course book for learners of English I have to use (New Total English, Intermediate, Student's Book, page 57) says:
'For First Conditionals, we use when rather than if to show that something is 100 percent certain to happen.'
And an example appearing in the exercises where this rule is applied reads:
'I will miss everyone when I leave my job.'
which should then mean
'I will definitely miss everyone / I am sure of missing everyone / I am sure to miss everyone if I leave my job.'
Whether or not I leave it? Or does the sentence necessarily imply that I will definitely leave my job at some point in the future?
What is '100 percent certain to happen': my missing everyone or my leaving the job or both?
If it is both, as I suspect, then isn't the clause a simple time clause, not an if clause, since there is nothing hypothetical about it?
Best Answer
Case 1: you have made up your mind that you are about to leave. Better still, even others are aware that you are about to leave. In other words, there is no uncertainty in your intention, and others' understanding that you will be leaving soon. In this case - when works.
Case 2: There is a possibility that you might leave. You are still uncertain in your mind whether you want to leave, or whether you will get the opportunity to leave. In this case - if works.
That's how the uncertainty factor mentioned by you plays out. With language you don't just communicate what's in the words, but you also offer interpretations for multiple other things that are not in the words. Both sentences are correct in that they express that you will miss them - but if you say if I leave, you are also telling the audience that you are not sure yet, and if you say when I leave you are indirectly making it clear to them that the decision has been taken.