Comma before ‘that’ and ‘which’

What sets English apart from most other languages is its use of the comma before a dependent (subordinate) clause. Dependent clauses (clauses introduced by words like “that”, “which”, “who”, “where”, “how”, etc.) are usually neither preceded nor followed by a comma. For example:

Cars that don’t have seat belts aren’t allowed to carry children. (correct)

Cars, that don’t have seat belts, aren’t allowed to carry children. (wrong)

I don’t know which one I want. (correct)

I don’t know, which one I want. (wrong)

Could you tell me where it is? (correct)

Could you tell me, where it is? (wrong)  

Dependent clauses are (and in fact must be) separated with commas only when the information contained in the clause is not important for the overall meaning of the whole sentence. A good way to recognize such clauses is to try to enclose the clause in parentheses; if the sentence still makes sense, you should use commas (or parentheses) to separate the clause from the rest, e.g.

Brazil nuts, which you can buy in a supermarket, are a great source of selenium.

Commas were appropriate here because we could replace them with parentheses:

Brazil nuts (which you can buy in a supermarket) are a great source of selenium.

Not using commas or parentheses would be a mistake in this case. The sentence

Brazil nuts which you can buy in a supermarket are a great source of selenium.

implies that only Brazil nuts sold in a supermarket (and not elsewhere) are a great source of selenium, which is certainly not the case.

Notice how the three examples we used at the beginning wouldn’t make sense if we put the dependent clause in parentheses:

Cars (that don’t have seat belts) aren’t allowed to carry children. (wrong)
I don’t know (which one I want). (wrong)
Could you tell me (where it is)? (wrong)