People often mistakenly use the pronouns "that" and "which" interchangeably. However, a grammar rule determines when a writer should use one relative pronoun or the other.
The rule is use "that" ONLY with essential (also called restrictive) clauses. Then, the rule says "which" mostly should be used with nonessential (also called nonrestrictive clauses), but it also can be used it with restrictive clauses.
So, what's the difference between a restrictive and non-restrictive clause? The sentences below show you.
Restrictive Clause: The study revealed a formal policy THAT provides a way to report HR violations anonymously is needed in many small agencies.
Essential (restrictive) clauses are necessary parts of a sentence while nonrestrictive (nonessential) clauses are not a core part of a sentence. Commas are placed around a nonrestrictive clause to show it is not part of the main sentence, as indicated below:
Nonrestrictive Clause: Several researchers theorize about the lack of accountability at the highest levels of government agencies, WHICH leads to the gaps in oversight.
Notice a comma precedes "which" in this sentence, indicating what comes after the comma is not a necessary part of the sentence. If a writer wants to make that information a part of the core sentence, then the writer should delete the comma and change "which" to "that."
When determining whether to use "that" or "which," identify if the words after the pronoun are part of the core sentence. If they are, you most likely should use "that." If the words are not a part of the core sentence, you definitely should not use "that."
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