word choice - When are "if" and "whether" equivalent?


Are if and whether equivalent in sentences like the ones below?



How to determine if my saddle is too high?
How to determine whether my saddle is too high?




We should check if everything is okay now.
We should check whether everything is okay now.




Answer



In general, if you're turning a question into a noun, whether tends to be preferred in formal use. So this includes your example plus, e.g.:



The discussion was about [whether climate change was an important issue].


The issue is [whether we need to act now].



In informal usage, if is also possible in these cases.


It's important to understand that 'whether' and 'if' are not always interchangeable. The word whether only has the meaning of "if ... or not" (though you can still use the formula whether ... or not for emphasis), so for example you can't use whether in this case:



If/*whether you decide to come, give me a call.



Another key difference is that whether can introduce an infinitive, whereas if can't (they're different parts of speech):



They couldn't decide whether/*if to come.



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