Determining whether to use “that” or “which” in a sentence can be a challenge. Sometimes, it’s obvious…
If you’re discussing what option to pick from among multiple options, “which” is correct.
Ex: “Which hat should I buy?”
If you’re indicating/identifying a specific, finite object, “that” is correct.
Ex: “I will buy that hat.”
However, it can appear more confusing if the sentence is more complex, or phrased unusually. But, it’s not actually more confusing – there’s a quick, easy rule to help determine when to use which option (use “which” there because we’re talking about multiple options!). In complex sentences, that/which usually are used with clauses. If you can remove the clause without altering the meaning or reducing the clarity of the sentence, then “which” is probably correct; otherwise, “that” is probably correct.
Ex. 1: “I will buy the hat that is green.”
Explanation: this sentence specifies, with a clause, that I am specifically buying the green hat. If that aspect of the sentence is removed, then necessary information is lacking (the person I’m speaking to will no longer know which hat I mean!). Grammatically, this is called a defining (or restrictive) clause – it’s a clause that defines the thing being described, and marks it as “this specific thing (as opposed to any other thing).”
Ex. 2: “I bought the hat, which is green, to wear to school.”
Explanation: the clause is an interjection which is not necessary to convey meaning, it simply add flavor. If it’s removed, the essential point of the sentence (that the hat was purchased to wear at school) remains. Grammatically, this is called a non-defining (or nonrestrictive) clause – a clause that isn’t necessary to define the thing, and when removed won’t have a major impact on the reader’s ability to know which thing is being described – “I’m interacting with this thing (and, coincidentally, this thing has this trait).”
So, just remember: if the clause is essential to conveying the full meaning of the sentence, use “that.” If the clause can be removed without impacting the sentence’s meaning, use “which.”
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