Must is a word that expresses necessity or obligation, telling us that something is required or has to happen. When your teacher says “You must finish your homework before recess,” she means it isn't optional. When a recipe says “The butter must be softened,” it means the recipe won't work properly otherwise.

The word appears in rules and laws: “Drivers must stop at red lights.” It shows up in logical reasoning: “If Sarah borrowed my book and it's not on my shelf, she must still have it.” And it expresses strong recommendations: “You must read this book, it's amazing!”

Must carries more weight than words like “should” or “ought to.” Those suggest something would be a good idea. Must says there's no real alternative. When a sign reads “All visitors must check in at the office,” that's not a friendly suggestion.

People sometimes use must to express certainty about something they've figured out: “The package must have arrived today” means the speaker is confident it did, based on the evidence. Scientists use similar logic: “If the experiment produced these results, the temperature must have been exactly right.”

As a noun, a must is something you really need or strongly recommend: “Water is a must on a long hike” or “That movie is a must-see.”

The opposite is must not (or the contraction mustn't), which means something is forbidden or impossible: “You must not run in the hallway.”