In everyday speech, people often use “if” and “whether” interchangeably. In fact, many consider the following sentences equivalent:

A) Nigel doesn’t know if he’s fired.

B) Nigel doesn’t know whether he’s fired.

On GMAT sentence correction questions, sentence B is preferable to sentence A.

On the GMAT, use “if” in a conditional sentence to show that one thing will happen if something else happens. Use “whether” to show that two alternatives are possible.

For me, the easiest way to determine which word to use is first to recognize a useful feature of conditional sentences: They still make sense if you reverse their order.

Take the conditional sentence, “If Hank wins the lottery, he will buy a yacht.” If we reverse the order, we get, “Hank will buy a yacht if he wins the lottery.” Since both sentences make sense, and both express the same idea, we have a true conditional. So, using “if” is appropriate.

Similarly, we can take the conditional sentence, “Carl will scream if he sees a spider,” and reverse the order to get, “If Carl sees a spider he will scream.” Both sentences express the same idea, so using “if” is appropriate.

Now let’s try this technique with the sentence, “Nigel doesn’t know if he’s fired.” The word “if” suggests that this is a conditional sentence. To test this, we’ll reverse the order to get, “If Nigel is fired, he doesn’t know.” Hmm, that doesn’t make any sense.

The original sentence should read, “Nigel doesn’t know whether he’s fired.” The word “whether” indicates that two alternatives are possible. Nigel may or may not be fired.

Let’s try one more.

A) Bill cannot remember if he locked the door.

B) Bill cannot remember whether he locked the door.

To test whether sentence A is a true conditional (requiring “if”), we’ll reverse its order. We get, “If Bill locked the door, he cannot remember.” This makes no sense, so sentence B must be the best answer.

So that’s how the GMAT handles sentences with “if” and “whether.” Please note that the rules shown here are not universally accepted. They are, however, accepted on the GMAT.