As a relative pronoun, "which" introduces a relative clause (also called an adjective clause) that provides additional information about a preceding noun or noun phrase. The crucial distinction here is whether the information is essential (restrictive) or non-essential (non-restrictive) to the meaning of the sentence.
A. Non-Restrictive Clauses (Non-Essential Information)
This is the most common and generally accepted use of "which" in American English.
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Purpose: The clause introduced by "which" provides extra, non-essential information about the noun. If you remove the clause, the main meaning of the sentence remains clear.
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Punctuation: Non-restrictive clauses are always set off by commas. If the clause is in the middle of the sentence, it will have a comma before and after it.
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Refers to: Typically refers to things or animals, not people.
- Example: "My car, which is a blue Ford Focus, was stolen last week." (The information "which is a blue Ford Focus" is extra. The main point is "My car was stolen last week," and the listener already knows which car you're talking about.)
- Example: "The old schoolhouse, which is one of my favorite historical sites, is in dire need of renovation." (The clause adds an interesting detail, but the sentence still makes sense without it: "The old schoolhouse is in dire need of renovation.")
- Example: "She seemed more talkative than usual, which was because she was nervous." (Here, "which" refers to the entire preceding clause/sentence.)
B. Restrictive Clauses (Essential Information) - Usage Note
While "that" is generally preferred for restrictive clauses in American English, "which" can sometimes be used, especially in British English, or in more formal contexts.
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Purpose: The clause provides essential information that is necessary to identify or define the noun it modifies. If you remove the clause, the meaning of the sentence changes or becomes unclear.
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Punctuation: Restrictive clauses are NOT set off by commas.
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Refers to: Typically refers to things or animals.
- Example (American English preference): "The book that I borrowed from her is worn." (Here, "that I borrowed from her" is essential to specify which book is worn. If you said "The book is worn," it wouldn't be clear which book you meant.)
- Example (British English/Formal - where "which" might be used restrictively): "The book which I borrowed from her is worn." (This would be grammatically acceptable in British English, though "that" is still common.)
Key Distinction: Commas are your guide!
- If you can remove the clause without changing the fundamental meaning of the sentence, use which (and surround it with commas).
- If the clause is essential for identifying the noun, use that (and do not use commas).
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