Q. When is it correct to use “if” and when is it correct to use “whether?” Thank you.
A. “Whether” should be used to introduce a noun clause: He asked whether his hat was on straight. It is especially helpful to use “whether” (1) when the noun clause begins the sentence: Whether the hat stayed on his head was of tremendous importance to him; and (2) when the use of “if” would result in ambiguity. “He asked me to tell him if his bald spot was showing” is often taken to mean that I should tell him whether it’s showing or not, whereas it literally means that if I can’t see his scalp, I can keep quiet. Because the popular meaning is different from the literal meaning, there is ambiguity. “He asked me to tell him whether his bald spot was showing” clearly means that I should tell him whether it’s showing or not. So use “whether” to introduce an alternative; avoid substituting “if” for “whether” unless perhaps you want to strike an informal tone, as in true-to-life dialogue.