Let's say you are giving advice to somebody who wants to go for a longer hike in the Alps.
Go back if the weather turns bad / changes.
Go back when the weather changes.
The meaning here is "as soon as", so it should be "when". However, I can't be sure that it will change. So "if" in the sense of "in case" would also be appropriate, wouldn't it?
when it changes means it will change. if it changes means it may or may not change. Clive
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when it changes means it will change.
if it changes means it may or may not change.
Clive