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I am struggling with understanding when I can use if/when interchangeably. The situation is like this, I meet my friend outside who is heading to the dentist. I say (in that situation, knowing he is about to see the dentist in a while):

If you are going to the dentist, you should brush your teeth.

But I can say that in general:

Children, if you are going to the dentist, you should brush your teeth.

Should it not be better/needed to use 'when' in any of these sentences? I still feel like 'if' means 'maybe it is going to happen' while 'when' 'it is going to happen at some point'.

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I feel if~should and when~must or before going to the dentist you should brush your teeth –  mplungjan Jul 16 '13 at 14:07
    
Well but isn't one for general situations (like saying) and one for (one-off situations)? I would say "When" implies general knowledge, statement that is still valid. –  user970696 Jul 16 '13 at 14:27

2 Answers 2

Well, yes, in this particular instance if/when are interchangeable. It is up to you how you would like to use them. You are correct. "If" implies it could happen, while "when" means that it is going to happen. For this type of sentence there isn't a word structure rule that states you have to use one or the other. It's a matter of preference for what you want to say.

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So if a friend tells me on the street "I am going to the dentist" I can say "When you are going to the dentist, you should brush your teeth"? And it is the same as with "If"? –  user970696 Jul 16 '13 at 14:35

In this specific context, if and when actually have slightly different connotations. As Kristen Ramos said, both are substantially correct; your meaning will be understood in either case. That said, the if-version is the preferred way to give advice about a specific, imminent situation.

If you are going to the dentist, you should brush your teeth.

means something like

If you are going to the dentist [right now, as I think you are], you should brush your teeth.

Here, the scope of the if (the ‘right now’) is understood from context. You don't mean (and a native speaker wouldn't mistake you as meaning) ‘if you're ever going to the dentist.’ In contrast,

When you are going to the dentist, you should brush your teeth.

is open to at least two interpretations, both of which are unnecessarily general for your situation:

1) When you are going to the dentist [whenever that happens to be], you should brush your teeth.
2) Whenever one is going to the dentist, one should brush one's teeth.

When is very frequently used in this second sense, to describe a general rule or principle. This means that

Children, when you are going to the dentist, you should brush your teeth.

is the preferred form for your second example, because the speaker is dispensing broad advice not tied to a particular situation.

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