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?