That or Which?
"That" and "which" both introduce clauses that describe the preceding noun, but they're not the same. "Which" is preceded by a comma and is used with a nonessential description, while "that" gets no comma and is used with essential descriptions. For example: She chose the only white peach that was ripe. (There were several peaches, but only one was ripe.) She chose the only white peach, which was ripe. (There was only one white peach, and it happened to be ripe.)
Wrong: Many men want to be the spy, James Bond.
Why: There's a comma between a noun and its restrictive form of identification.
Right: Many men want to be the spy James Bond.
Why: This has to do entirely with the meaning of the sentence, which is that these men want to be the spy named James Bond, not that the men want to be the spy. Which spy? When you place a comma before an identifier, phrase or clause, you're saying it can be removed from the sentence without changing the meaning -- that it's nonessential. In this case, "James Bond" is essential, so there's no comma before it.
Consider the following examples to see how commas can change a sentence's meaning:
Sentence: The children, who couldn't do the math problem, stayed after class for tutoring.
Means: All of the children stayed after class. (The "who couldn't do the math problem" bit is preceded by a comma, so you can remove it without changing the meaning.)
Sentence: The children who couldn't do the math problem stayed after class for tutoring.
Means: Only the children who were having trouble stayed after class.
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