I was doing a grammar activity and filling in sentences when I came across the instruction: "fill in when or if."
I was wondering, since these two can be used in a lot of same sentences, what is the difference between sentences like this?
Chris might call while I'm out this evening. [If/When] he does, can you take a message?
Best Answer
If you say when something happens, you imply it definitely will happen, even if the precise timing is unknown. You use if when there's uncertainty about whether the event will happen at all.
It's quite common — especially in informal contexts — to use when/if as a shorthand way of signifying when, but allowing for the possibility that the event in question may not in fact occur.
Personally I tend to use the composite when/if rather than if/when, for no reason I can clearly explain. I can't easily search for those hyphenated forms to check others' usage, but I can use NGram to check the more extended equivalents. This covers well over a million instances, so I think it's statistically significant. But I have no idea why three out of four people put them the opposite way round to me...