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Four Word Pairs You're Probably Using Wrong

Pop Quiz: Each of the following questions contains an often-confused word pair. Pick the correct word in the context of each sentence.

  1. I’m looking for a car (that, which) can get at least 35 MPG on the highway.

  2. How will the news (affect, effect) the outcome of the election?

  3. The bottled water in your hotel room is (complimentary, complementary).

  4. I wanted to (ensure, insure) that dinner would be ready by 6 p.m.

Ok, let’s see how you did.

That vs. Which

In order to distinguish these word pairs, you need to first understand essential vs. non-essential phrases. “That” is used for essential phrases.

As the name suggests, an essential phrase is a part of a sentence that is essential to the meaning. In other words, if you took that phrase out, the meaning of the sentence would be altered. Here’s an example:

The car that she drives to work is a Toyota. The car that she drives to the country is a Subaru.

The two “that” phrases—“that she drives to work” and “that she drives to the country”—are essential to the meaning of the sentences. Without them, here’s what you’d have:

The car is a Toyota. The car is a Subaru.

“Which,” on the other hand, is for non-essential phrases—phrases that provide some extra information but could be removed entirely without altering the meaning of the sentence. To further emphasize the parenthetical nature of the non-essential phrase, “which” phrases are always set off from the rest of the sentence by commas. Here’s an example:

Susan’s car, which is a 2004 Ford, is parked in the long-term lot.

In this case, the point of the sentence is that Susan’s car is parked in the long-term lot. The make and year are extra details.

So what’s the correct answer to No. 1? I’m looking for a car that can get at least 35 MPG on the highway.

Affect vs. Effect

This one is a little more straight-forward. “Affect” is a verb and “effect” is a noun. Here’s what they look like when used in a sentence:

How will global warming affect our environment over the next 20 years?

Global warming is expected to have an effect on our environment.

In the quiz question above, we’re looking for a verb, so the correct answer is: How will the news affect the outcome of the election?

Compliment vs. Complement

You’ll probably need to just memorize definitions to keep these two words straight.

Compliment with an “i” is an expression of praise, as in: He complimented her on her writing talent. It can also refer to a free gift, such as a complimentary mint presented at the end of a meal.

Complement with an “e” means to make something complete, as in: The belt complemented my outfit.

Based on that, what’s the answer to No. 3? The bottled water in the hotel room is complimentary.

Ensure vs. Insure

We see this error all the time, but the rule is quite straight-forward: Insure is used only in cases where you're talking about the monetary kind of insurance, like auto insurance or health insurance. Most of the time you probably mean ensure, as in: I wanted to ensure that dinner would be ready by 6 p.m.

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