Correct Usage of Which and That in Sentences

Improve English Skills by Using “Which” and “That” Correctly

Some people are unsure when to use "which" compared to "that;" however, they can make the correct choice if they understand restrictive and nonrestrictive clauses.

When you buy research papers, pay attention that writers, as well as speakers, incorrectly choose “which” when the correct choice is “that,” or vice versa; but they can avoid making this common mistake by learning the role adjectives play within sentences and the difference between restrictive and nonrestrictive adjective phrases and clauses.

The Role of Adjectives in the English Language

Adjectives describe and/or modify nouns or pronouns by answering certain questions:

• Which one?
• How many?
• What kind?
• Whose?

Adjectives also include the articles “the,” “a,” and “an,” which point to specific nouns, “the” to those beginning with consonants, “a” and “an” to those beginning with vowels.

• Walking into the garage at night, Tom thought he smelled gas.
• Since the light bulb had blown, Tom lit a match so he could see.
• What followed was quite an explosion.

Adjective Clauses and Phrases in the English Language

Not only individual words act as adjectives but also phrases and clauses. For example, look at the following sentences:

Tom is tall and muscular. (Tall, muscular = predicate adjectives modifying “Tom”)
A short, thin woman, Sue is Tom’s polar opposite. (Short, thin = adjectives modifying “woman”)
Tom wants to buy the TV on sale at Wal-Mart. (On sale at Wal-Mart = adjectival prepositional phrase, telling which TV Tom wants to buy)
The TV that Tom wants to buy is on sale this week at Wal-Mart. (That Tom wants to buy = adjective clause, also telling which TV Tom wants to buy)
Note: Phrases are groups of words that add meaning to sentences, but unlike clauses, they do not contain subjects and verbs.

Nonrestrictive Adjective Clauses and Phrases in English

Adjective clauses and phrases can be either nonrestrictive or restrictive. They are nonrestrictive when used simply to describe a noun or pronoun, and nonrestrictive clauses and phrases can be omitted without detracting from the meaning of the sentence, for example:

Tom would like to buy term paper, which he sees as a necessity, but Sue said they couldn’t afford one and nixed the idea.
The adjective clause (in italics) in the sentence above is merely additional information that could be omitted without changing the meaning of the sentence.

Here is another example:

Tom thought his latest novel, an adventure tale about vampires, would be an instant success.
The adjectival appositive phrase (in italics) in the above sentence is merely additional information that the writer is sharing, not essential to the meaning of the sentence, and, therefore, set off with commas.

Restrictive Adjective Clauses and Phrases in English

Restrictive adjective clauses and phrases are essential to the meaning of the sentence because they modify a noun or pronoun by telling which one (or ones); therefore, these clauses and phrases are not set off by commas and cannot be omitted from the sentence, for example:

Tom wants to purchase the surround-sound Dolby system that is on sale at Wal-Mart because he thinks it is a good price and well within his and Sue’s budget.
In the above sentence, the adjective clause is necessary because Tom does not want to purchase just any sound system; he wants to purchase the system that is on sale at Wal-Mart. If the clause “that is on sale at Wal-Mart” were removed, the reader would no longer know to which system the writer was referring.

Here is another example:

While exploring his mother’s attic, Tom found a pile of comic books dating back to his high school years.
In the sentence above, “dating back to his high school days” is a restrictive participial phrase because it relates the comic books to a specific time period, and without this phrase, the reader would know only that Tom had found a pile of nondescript comic books.

The Difference Between “Which” and “That”

The important thing to remember is that whereas the word “that” is considered a defining or restrictive pronoun, the word “which” is considered a non-defining or nonrestrictive pronoun. Therefore, although there are exceptions, one should normally use “which” for nonessential information, setting the clause off with commas, and “that,” with no commas, for essential information; for example:

Tom told Sue that his new novel, which was destined to be a best-seller, was titled Love and Betrayal in Transylvania.
Tom told Sue the novel that was destined to be a best-seller was Love and Betrayal in Transylvania.
In summary, writers, as well as speakers, can avoid misusing these two words if they take time to learn the difference between nonrestrictive and restrictive clauses and phrases.

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Useful Resources:

Editing Writing for Common Word Choice Mistakes

Grammar: The Dos and Don'ts of Apostrophes

How to Become a Certified Copyeditor