To suppose means to imagine something might be true, or to consider a possibility without being certain about it. When your teacher says, “Suppose you had a million dollars,” she's asking you to think about a situation that isn't real yet. When you say, “I suppose we could try that,” you're agreeing tentatively, without total confidence.

The word carries a sense of uncertainty or possibility rather than certainty. You might suppose it will rain tomorrow based on dark clouds, but you don't know for sure. Scientists suppose how something works, then test their ideas through experiments. When your friend doesn't show up and you say, “I suppose she forgot,” you're making your best guess about what happened.

People also use suppose when they're thinking through problems: “Suppose we only have two hours. How many chapters can we finish?” This kind of supposing helps you plan and make decisions.

Supposed to has a different meaning: it refers to what's expected or required. When you're supposed to clean your room, that's what you're expected to do. When the bus is supposed to arrive at 3:00, that's when it's scheduled to come. If something is supposed to be good but disappoints you, it means people expected it would be good, but it wasn't.