When you say something is supposed to happen, you mean it's expected, planned, or required to happen. If you're supposed to clean your room before dinner, that's what your parents expect you to do. If a package is supposed to arrive on Tuesday, that's when it's scheduled to come.

The word often appears when reality doesn't match expectations. You might say, “The experiment was supposed to create a chemical reaction, but nothing happened” or “We were supposed to have a field trip today, but it got canceled.” Notice how supposed to points to a gap between what should be and what actually is.

People also use supposed to express doubt about whether something is really true. If someone tells you about a “supposedly haunted house,” they're suggesting that some people claim it's haunted, but maybe it isn't. When you say, “He's supposed to be the fastest runner in school,” you're repeating what others say without necessarily confirming it yourself.

The phrase supposed to can also mean something is generally believed or accepted: “You're supposed to say 'please' and 'thank you'” describes a social rule, even though it's not a law. Understanding what you're supposed to do helps you meet expectations, though sometimes people have good reasons to question whether those expectations make sense.