On the GMAT, if indicates a conditional sentence. In a conditional sentence, the occurrence of one event (a condition) guarantees the occurrence of some other event. For example, in the sentence If it rains tomorrow, Hank will fire the woman who chews his food for him, the occurrence of rain guarantees a job termination.

Alternatively, the word whether indicates that two alternatives are possible. For example, in the sentence Belinda arrived late because couldn't decide whether to smell like Rihanna or J.Lo, the two possible alternatives are smelling like Rihanna or smelling like J.Lo.

Another way to help us determine whether to use if or whether is to recognize an interesting feature of conditional sentences: Conditional sentences make sense if you reverse their order.

Consider this conditional sentence: If a skunk walks into your courtroom, you have every right to yell "odor in the court." If we reverse the order, we get You have every right to yell "odor in the court" if a skunk walks into your courtroom. Since both sentences make sense, and both express the same idea, we have a true conditional. So, using if is appropriate in this situation.

Likewise, if we take the conditional sentence Don't bring a urine sample if the doctor didn't request one, and reverse the order, we get If the doctor didn't request a urine sample, don't bring one. Once again, the two sentences make sense, and they express the same idea, so using if is appropriate.

For more information on this topic, you might want to read an article for BTG: https://www.beatthegmat.com/mba/2013/11/ ... n-the-gmat

Cheers,
Brent