When do we use "which" or "that" in English?
Both words are acceptable in ordinary language, but the subtle differences between them are important in business, technical, and legal writing.
We use "which" and "that" when talking about things.
"That" is a restrictive word. It restricts us to a certain group of things.
For example, "Dogs that are naughty must stay outside".
Not all dogs must stay outside, just those that are naughty.
"Which" is an inclusive word. It includes all things in a specified group.
For example, "Dogs, which I detest, must stay outside."
The statement applies to all dogs, not just some of them.
We use commas to set off the clauses that start with "which".
Classes that are held on Wednesdays are in Elliott Hall (this applies only to Wednesday classes).
Chemistry classes, which involve labs, meet in Elliott Hall, Room 208 (This applies to all chemistry classes, whatever the day of the week).
The pies that were blueberry looked delicious (there were other pies, but they didn't look quite as delicious).
The pies, which were blueberry, looked delicious (all the pies were blueberry).
I like the songs that you write but sad songs, which I detest, are another matter.
Rules of Thumb:
- If you want to include all members of a group, use "which"
- I you want to limit your description a subset of a larger group, use "that".