Grammatical Mistakes that Very Intelligent People Make - That vs. Which
This one is very tricky, and an extremely common mistake, so heed the rule: 'That' starts a dependent (also called restrictive) clause; 'which', an independent (nonrestrictive) one.
In the example, "The dog that chased me down the street was black and brown", the clause "chased me down the street" is an important part of the sentence. For something that is an additional piece of information, but incidental to the main idea, use 'which', as in: "The dog, which was black and brown, chased me down the street on my way to the bus." The independent clause starting with 'which' can be removed without hurting the most important information.
Sounds technical, but the easy way to remember this: If you need to use a comma, use which. If not, use that. And don't forget to add the second comma after the clause!
Another usage tutorial :
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