If/Whether

Here’s a finer point of grammar that I probably learned years ago. On page 763 of Infinite Jest, Mario is talking to his mother:

Mario: “How can you tell if somebody’s sad?”

Avril: “You mean whether someone’s sad.”

Not being able to recall the distinction between “if” and “whether”, I asked the question of the Infinite Summer forum and got this response from user isabella:

From Bryan A. Garner’s A Dictionary of Modern American Usage (which DFW discussed at length in “Tense Present: Democracy, English, and the Wars Over Usage”):

It’s a good practice to distinguish between these words. Use if for a conditional idea, whether for an alternative or possibility. Thus, Let me know if you’ll be coming means that I want to hear from you only if you’re coming. But Let me know whether you’ll be coming means that I want to hear from you about your plans one way or the other.

Copyeditors (I am one) generally try to maintain the distinction.

Voilà! The Wallace essay is online here.