Which do you use? That one!

The answer is simple: In a defining clause, use “that.” In a non-defining clause, use “which.” In other words, “which” and what comes after it is disposable. If you can take the whole clause out without destroying the sentence, you can use “which.”



Dr. Rafa Almodovar requested a clipboard, which helped him take notes. 

(Here, the “which helped him take notes” is just a description that can be removed if necessary.)

The clipboard that has a pen attached is hanging on the wall, ready for Dr. Rafa Almodovar. 

(Here, we imply that there is another type of clipboard. The “that has a pen” piece specifies the clipboard, and removing it would take a key piece of information out of this sentence.