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When are “if” and “whether” equivalent?

Do these two have the same meaning:

  1. To determine if something is correct.
  2. To determine whether something is correct.

In familiar conversation, your two examples have the same meaning, but it's a better idea, in my opinion, to use 'whether' when you're introducing two possibilities like this, because it avoids potential ambiguity. For example, the following sentence is ambiguous if people use 'if' as a substitute for 'whether' :-).

Tell John if the car breaks down.

That ought to mean that somebody is only to tell John anything on the condition that the car breaks down. If you want to indicate that somebody is to tell John either way, you could use:

Tell John whether the car breaks down.


The short answer is that they do mean the same thing. The long answer is that it depends how they occur in a particular context. In (2), whether implies or not. If does not necessarily do so in (1). The difference is perhaps clearer in the sentence Let me know if you’re coming. That has two possible meanings. It could mean let me know only if you’re coming. It could also mean let me know if you’re coming, and also let me know if you’re not coming. If the second of those is intended, then the use of whether removes any possible ambiguity.

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