"It is doubtful whether he is telling the truth"
Why is it not possible to use "if" in this case? Thank you in advance.
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It is doubtful [whether/if he is telling the truth]
Both "whether" and "if" are possible.
Dubitatives like this are rather weird. Syntactically the bracketed element in your example is an interrogative complement clause (indirect question), but semantically it does not express a question at all but simply a proposition, and is thus equivalent to a declarative.