That is used when the word introduces a clause essential (or restrictive) to the meaning of the sentence. That helps identify the information and is not set off by commas.
- This is the Chevy that has a new engine. Here is the rule that applies in both cases.
Which is used when the word introduces a clause not essential (or nonrestrictive) to the meaning of the sentence. Which helps amplify the information and is set off by commas or dashes.
- My Chevy, which runs well, has a new engine.
- This rule, which became effective in 2011, applies to you.
Memory hook: That identifies and which amplifies.