“I don’t know if you will find this post helpful”
Do you spot the error in the preceding sentence? This error is common in casual spoken English, but it will cost you on the GMAT Sentence Correction. In that sentence, the word “if” is incorrect: it should be replaced by the word “whether.”
When to use “if”
The word “if” is used for clauses that specify conditions or speculate on something hypothetical.
1.) Condition: “If you finish your peas, you can have dessert.”
2.) Hypothetical: “If I regularly ate my vegetables, I probably would be healthier.”
In formal logic, the clause following the “if” clause would begin with the word “then”: that’s perfectly acceptable grammatically, but not at all necessary. For example, in both of those sentence, the word “then” could be inserted right after the comma, and would add a bit of emphasis to the logical relationship, if that were something that needed underscoring.
The last clause of the previous paragraph highlights a particular category of conditional statements, those using the subjunctive. For more on the subjunctive mood, see this post. The GMAT loves “if”-clauses involving the subjunctive.
When to use “whether”
The word “whether” is a relative pronoun, which means it introduces a relative clause. A “whether” clause is always about the uncertainty in a choice or alternative, and the clause itself may stand apart from the sentence, the way an “if” clause does, or may act as a noun. When it stands apart, it is like an “if” clause in which the definite causal nature has been replaced with uncertainty or irrelevance. When it acts as a noun, the clause may act as the subject of the sentence, or as the object of an epistemological verb (to know, to wonder, etc.) or a volitional verb (to care, to prefer, etc.)
3.) Whether you study French or Spanish, you will encounter an unfamiliar language in Japan.
4.) Whether or not I get the raise, I am going to buy that new car.
Notice, in either of those: if we removed the uncertainty of the choice, we could replace the word whether with the word “if” to get a more definitive conditional statement. Without making those changes, the word “if” would be wrong.
Subject of sentence:
5.) Whether you like jazz will influence your opinion of this new club.
6.) Whether I walk on her left or right side matters a great deal to her.
Object of an epistemological or volitional verb:
7.) I don’t know whether there is intelligent life elsewhere in the Universe.
8.) He doesn’t care whether you serve broccoli or Brussels sprouts with dinner.
In sentence #5-8, the word “if” would be 100% incorrect. The GMAT Sentence Correction loves to test that particular mistake.
Whether . . . or not
The word “whether” implies a choice, at least a pair of alternatives. Sometimes that choice is made explicit (as in sentences #6 and #8), and sometimes it is implicit (as in sentences #5 & #7). When the choice is implicit, is it grammatically correct to add the words “or not” after whether?
When the “whether” clause acts as a noun, the words “or not” add absolutely nothing to the sentence. Consider:
5a.) Whether you like jazz will influence your opinion of this new club.
5b.) Whether or not you like jazz will influence your opinion of this new club.
The meaning of both sentences is exactly the same. The second sentence adds two more words that contribute zilch to the overall meaning of the sentence. What is GMAC’s opinion of tossing in extra words that lengthen the sentence and contribute bupkis to the meaning? As you may well guess, they frown on these. Don’t expect to see “whether or not” in any correct GMAT SC answer choice when the clause is used as a noun.
When the clause stands apart, as in sentences #3 & #4, that’s another matter. In that construction, the alternative must be made explicit. In #3 there already was an explicit comparison of the two languages, but in #4 we absolutely must include the words “or not” after the word “whether”: the grammatical construction demands it. This is the only case in which the words “whether or not” could be correct on GMAT sentence correction.
Whether or not you like it, knowing the correct use of “whether” and “if” is important for GMAT Sentence Correction. If you can master these distinctions, you will perform well on a question that that befuddles many.
Two relevant SC questions in the GMAT Official Guide, which appear as:
a.) #34 & #75 in OG12e, and
b.) #34 & #78 in OG13e