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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?

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Ty for editing Sim. – Phonics The Hedgehog Jul 21 '11 at 23:21
    
Related If and when, when and if – Kit Z. Fox Jul 22 '11 at 1:00
    
@ChaosGamer FYI read the FAQ about tagging people's names with an "@" when you want them to receive a notification in their inbox. – Rachel Jul 22 '11 at 1:21
up vote 6 down vote accepted

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... .

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another great answer from @FumbleFingers. "If" carries a connotation like something MAY happen, but there is a chance that it may not. "When" denotes that it WILL happen, it is just a matter of at what point in time the event will carry itself out. – Rachel Jul 22 '11 at 1:19
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The technical term is that when presupposes that it will happen, while if merely assumes possibility. Presuppositions are a big deal, since that's how we manipulate social belief, and their traces are all over English and every other language. – John Lawler Jun 13 '13 at 16:20
    
I wish you would include John Lawler's comment in your answer. I found it most helpful. – Mari-Lou A May 23 at 8:35
    
@Mari-Lou: I think if the basic question as per OP's title were posted today, I'd vote to migrate to ELL rather than answering. I bet you didn't need the actual answer to that question anyway, but John's comment & link is certainly an interesting digression into "related concepts" from a true linguist's perspective. The basic question is entirely covered by my first paragraph (the rest being just my attempt at an interesting digression! :). But I doubt the mods would ever delete his comment - so good as it is, I see no real need to copy it into the answer. – FumbleFingers May 23 at 17:36

if is used to indicate a condition of for an event to take place while when is used to indicate time and it is also mostly use to ask questions. Like: When did you arrived

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When you are sure that something will happen use when. Not sure use if.

For instance: your mother tells you that tomorrow you are going to town. When she instructs you of what to do, she will say:

When you are going to town tomorrow buy me sugar.

But when she asks you or your brother that either of you will go town, she will say:

If you go town tomorrow, buy me sugar.

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Use "WHEN" for something that will certainly happen, if not use "If".

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IF-THEN

Causal adverbs determining antecedent and consequent:

If the Panthers win the tournament, then the Broncos lost the final.

(The antecedent and the consequent need not be related in time!)

WHEN-THEN

Temporal adverb specifying the time

When the sun rises then it is not dark outside anymore.

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