Couldn't is a shortened form of “could not,” used when something was impossible or when someone lacked the ability to do it. When you say “I couldn't reach the top shelf,” you mean you tried but physically weren't able to. When someone says “I couldn't finish my homework,” they might mean they ran out of time, didn't understand it, or something prevented them from completing it.

The word often appears when explaining why something didn't happen: “We couldn't go to the park because it was raining” or “She couldn't hear the teacher from the back row.” Sometimes people use it to express strong feelings about something that seemed wrong or impossible to accept, as in “I couldn't believe my eyes when I saw the sunset” or “I couldn't stand watching my friend be treated unfairly.”

You'll also hear couldn't in the phrase “couldn't care less,” which means someone has absolutely no interest in something. If your brother says he couldn't care less about which movie you watch, he's saying he has zero preference. Interestingly, people sometimes say “could care less,” which technically means the opposite, though both phrases are widely understood.