The problem in choosing between “which” and “that” arises from not using them based on the type of clause they are in.
If the clause is essential to the meaning of the sentence, use “that” and no comma.
If the sentence can be understood without the clause, then use “which” preceded by a comma.
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Copyright © 2007
Donna K. Reeder
Consider how the following line by Terry Pratchett changes meaning if we substitute “that” for a comma followed by “which”
“In the beginning, there was nothing,
which exploded.”
“In the beginning, there was nothing
that exploded.”
“This word [“which”], used immoderately, is possibly responsible for more bad sentences than any other in the language. Small wonder that James Thurber wrote: ‘What most people don’t realize is that one which leads to another.’
The simplest statement of it is this: if you see a which without a comma before it, nine times out of ten it needs to be a that. The one other time, it needs a comma. Your choice, then, is between comma-which and that.
Use that whenever you can...if you see a which with neither a preposition nor a comma, dash, or parenthesis before it, it should probably be a that.”
~Bryan Garner,
The Oxford Dictionary of American Usage and Style