Many people are confused about when to use “which” and when to use “that.” Often times, it is a subjective call on the part of the writer.
“Which” means the words following are not essential to the meaning of the sentence. In other words, the information adds a new element the reader may not need.
Example: We published the information, which was required by law, in the annual report.
Note: There is always a comma in front of “which” and always a comma at the end of the idea it is introducing. (Your grammar check – if it is turned on – will put a green squiggle under “which” to remind you to insert the comma in front, if you haven’t done so. Unfortunately, it cannot tell you where to place the second comma. Don’t forget to add it.)
“That” is used when the information following is essential to the meaning and to the reader.
Example: We published the information that is required by law in the annual report.
Note: There is no comma in front of “that.”
Some editors believe a sentence will be smoother if you remove “that” when it is not needed. ?Example: We published the information required by law in the annual report.
Personally, I delete ”that” whenever I can. It is usually just fluff. If you are uncertain, I suggest you read the sentence to yourself. If it sounds better with the word omitted, remove it. If you need it for rhythm or clarity, let it remain.