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The difference between “that” and “which” in relative clauses is that “that” is used for essential information while “which” is used for non-essential information. “That” is used for inanimate objects and people are usually referred to with “who”. When in doubt, try removing the relative clause to see if the sentence still makes sense.
There is a subtle but important difference between using that and that in a sentence, and it mostly has to do with relevance. Grammarians often use the terms “restrictive” and “nonrestrictive” when dealing with relative clauses. A relative clause provides additional information about the noun it describes, but could be considered relevant or irrelevant to the general point of the sentence. In other words, a restrictive relative clause, often starting with that, is usually considered essential or restrictive. Relative clauses starting with which may contain non-essential information and would be considered non-restrictive.
What might be more helpful in a person’s day-to-day search for correct word usage are actual examples that contain that or that in their correct settings. What people need to keep in mind is the idea of restrictive or non-restrictive clauses, which translates to “necessary” or “useful but not essential”.
In a sentence like “The company that invented the microchip we use invited us to a demonstration,” the word “that” is relative to “company.” There are thousands of companies in the world, but it’s important to know that “the company that invented the microchip” is especially the one holding the demonstration. In this sense, the relevant clause starting with that would be considered restrictive, since it is essential information necessary to identify the company. A person wouldn’t write, “The company that invented the microchip invited us to a demonstration.” in formal writing.
Which could be used in a similar sentence constructed like this: “Widgets Incorporated, which invented the microchip we use, has officially filed for bankruptcy.” In this sentence the related clause “who invented the microchip we use” is separated by commas. The microchip information is useful, but not essential to the main idea of the sentence. It could be removed and the sentence would still make sense. If the relative clause can be removed without changing the meaning of the sentence, it would be considered non-restrictive. Readers can identify the specific topic, in this case Widgets Incorporated, so “which” would be the correct pronoun to use. “That” is almost always used to trigger a non-restrictive relative clause, which can be remembered as a parenthetical sentence.
A sentence like “Widgets Incorporated, which invented the microchip, has filed for bankruptcy.” that would not be correct. “That” does not usually trigger a non-restrictive clause, as it is more closely related to the noun it modifies. The word “that” is also rarely used in reference to people. “The man who lost his hat yesterday found it this morning” would not be correct. “The warehouse that once stood on the corner has collapsed” would be correct, since “that” modifies an inanimate thing and the related clause “that once stood on the corner” is restrictive and essential.
It might be tempting to use “that” instead of “that” in a similar sentence, such as “The building that was on the corner was torn down,” but that wouldn’t be correct. Information about the location of the building is essential, so a restrictive clause would be required. A correct sentence using “which” would read like this: “The Olsen Building, which was located at the corner of 12th and Vine Streets, was demolished.” The sentence could still be understood without the non-restrictive clause.
In short, whenever the information is essential to identify the subject, the proper pronoun to use is that. If the information is not essential, or can be separated with commas, then the pronoun that is more likely to be correct. As with English grammar rules, however, there will be some exceptions to this rule. When in doubt, writers can try removing the relative clause and wondering if the sentence sounds complete and informative. Some grammar experts suggest silently adding the words “about” after that to determine if the information is relevant or irrelevant. If “by the way” seems to fit, the clause is not restrictive and should be marked with commas.
If the meaning of the sentence would be lost without the information then it is most likely restrictive and that would be the correct pronoun to use.
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