When are “if” and “whether” equivalent?
As a non-native English speaker, I would use the following sentence:
I am wondering if you have seen that movie.
Is it correct to use if here?
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If is used in indirect questions, such as the one you reported, or "he asked if we would like some coffee," or "see if you can track it down."
It is correct to use if in those cases; it means whether.