The choice between "which" and "that" in adjective clauses is a common point of confusion in English grammar. The core distinction lies in whether the information in the clause is essential or non-essential to the meaning of the sentence.
"That": For Essential (Restrictive) Clauses
"That" is used to introduce a restrictive clause. A restrictive clause contains essential information that is necessary to identify the noun it describes. If you were to remove this clause, the meaning of the sentence would change significantly, or it would become too vague.
* Rule: Refers to things (and sometimes people, though "who" is often preferred).
* Punctuation: Restrictive clauses introduced by "that" are never set off by commas.
Examples:
* The book that is on the table belongs to me.
* The clause "that is on the table" is essential. Without it, the sentence "The book belongs to me" is vague. We wouldn't know which book.
* I want the phone that has the best camera.
* The clause "that has the best camera" restricts the options and identifies the specific phone you want.
* The car that I bought last week has already broken down.
* The clause "that I bought last week" is essential to specify which car we are talking about.
"Which": For Non-Essential (Non-Restrictive) Clauses
"Which" is used to introduce a non-restrictive clause. This type of clause provides extra, non-essential information about a noun that is already clearly identified. If you remove the clause, the core meaning of the sentence remains intact.
* Rule: Refers only to things.
* Punctuation: Non-restrictive clauses introduced by "which" are always set off by commas.
Examples:
* My new car, which is a hybrid, gets excellent gas mileage.
* The noun "my new car" is already specific. The clause "which is a hybrid" simply adds extra information. The main point is that the car gets good mileage.
* The Eiffel Tower, which is in Paris, is one of the most famous landmarks in the world.
* We already know what the Eiffel Tower is. The fact that it's in Paris is additional, non-essential information in this context.
* We had dinner at their house last night, which was a lot of fun.
* Here, "which" can refer to the entire idea or event of having dinner at their house.
Key Distinction: Essential vs. Non-Essential
The easiest way to decide is to ask: "Is this information necessary to identify what I'm talking about?"
* If YES → Use "that" (no commas).
* Sentence: Give me the box that is on the counter. (We need this information to know which box).
* If NO → Use "which" (with commas).
* Sentence: The box, which is made of cardboard, is on the counter. (The material is extra information; we already know which box).
Summary Table
| Pronoun | Type of Clause | Function of Clause | Refers To | Punctuation | Example |
|---|---|---|---|---|---|
| That | Restrictive | Essential information to identify the noun. | Things | No Commas | The pen that I lost was my favorite. |
| Which | Non-Restrictive | Non-essential, extra information. | Things | Commas | My pen, which has blue ink, is on my desk. |
A Note on Prepositions
When a preposition comes directly before the relative pronoun (a formal structure), you must use "which" for things, not "that."
* Incorrect: The theory about that he was speaking was complicated.
* Correct: The theory about which he was speaking was complicated.
* More Common/Less Formal: The theory that
he was speaking about was complicated.
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