If your teacher says, “You must hand in your project today,” you can decide not to do it, because it’s only advice.

Mustn’t and shouldn’t mean exactly the same thing in every situation.

We use should or shouldn’t when the speaker gives an opinion about what’s right or best.

“You mustn’t touch that button” and “You shouldn’t touch that button” are both correct, but the first sounds more serious.

“You shouldn’t talk with food in your mouth” is advice, but it’s often treated like a social rule.

You _______ tell lies to your parents. It’s not honest.

In this sentence: “You ___ tell anyone this secret,” the correct word is mustn’t, because it’s a strong prohibition.

Should can be used to give polite advice, while must is used for laws, rules, or very strong personal orders.