Here's why:
* Restrictive vs. Non-Restrictive Clauses:
* Restrictive clauses provide essential information that identifies the noun it modifies. They are not set off by commas.
* Non-restrictive clauses provide additional, non-essential information. They are set off by commas.
Example:
* Restrictive: "The car which is red is mine." (The "which" clause identifies which specific car is yours, making it essential information.)
* Non-restrictive: "The car, which is red, is mine." (The "which" clause provides additional information about the car, but it doesn't change the car's identity.)
In general:
* If the clause starting with "which" is essential to understanding the sentence, no comma is needed.
* If the clause starting with "which" could be removed without changing the sentence's core meaning, a comma is needed.
However, there are some exceptions:
* When "which" refers to a specific person or thing, it is often treated as a non-restrictive clause, even if it seems essential: "I met a man which is my father." (This is grammatically incorrect, as "which" should refer to a thing, not a person, and a comma is needed.)
* In formal writing, some writers use a comma before "which" even in restrictive clauses for clarity.
Ultimately, the best way to determine whether a comma is needed before "which" is to consider the sentence's meaning and whether the clause is restrictive or non-restrictive.