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'.
Best Answer
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.
means something like
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,
is open to at least two interpretations, both of which are unnecessarily general for your situation:
When is very frequently used in this second sense, to describe a general rule or principle. This means that
is the preferred form for your second example, because the speaker is dispensing broad advice not tied to a particular situation.