Which helps us ask questions or point to specific things within a group of choices.

When you ask a question, which helps narrow down options: “Which book should I read next?” or “Which flavor of ice cream do you want?” You're asking someone to choose from available possibilities, a particular book or flavor from the set in front of you.

Which also connects ideas in sentences, adding information about something you just mentioned. If you write “I finished the mystery novel, which had a surprising ending,” the word which introduces extra details about that specific novel. Think of it as a pointer that says “I'm about to tell you more about that thing I just mentioned.”

People sometimes confuse which with that, but there's a useful distinction. Use which when you're adding extra, bonus information (and usually put a comma before it): “The library, which stays open late on Thursdays, is my favorite study spot.” Use that when the information is essential to understanding which thing you mean: “The library that stays open late on Thursdays is my favorite study spot.” In the first sentence, you're adding a helpful detail. In the second, you're specifying which library you're talking about, distinguishing it from other libraries that close earlier.