WHEN AND IF "IF" is used to suggest that you are not sure that something will happen. "When" is used to talk about events that are certain or very likely to happen. I see you in September when I come back. (= I am sure I will come back) Perhaps I'll see you in September if I have time. (=Perhaps I won't have time) Are you going out later ? Perhaps . If I go out , I will close the window . (= It is possible that I will go out, but I am not sure.) Are you going out later ? Yes , I am . When I go out , I'll close the window . ( I am sure that I will go out.) In generalizations about things that happen repeatedly , both when and If are possible, without much difference of meaning. If/When you heat metal it expands. Conditionals Home Page
I see you in September when I come back. (= I am sure I will come back) Perhaps I'll see you in September if I have time. (=Perhaps I won't have time)
If/When you heat metal it expands.