Shouldn't is a contraction of “should not,” used to say that something is not a good idea or not the right thing to do. When you tell your little brother he shouldn't touch the hot stove, you're warning him about danger. When your teacher says you shouldn't wait until the last minute to start your project, she's giving advice about how to succeed.

The word often expresses a rule, recommendation, or warning. You shouldn't run with scissors. You shouldn't interrupt when someone else is talking. Sometimes it expresses regret about the past: “I shouldn't have eaten that third piece of cake” means you wish you had made a different choice.

Shouldn't is less forceful than “must not” or “cannot.” When someone says you shouldn't do something, they're usually giving guidance rather than issuing an absolute command. You shouldn't talk during the movie, but the staff probably won't throw you out for whispering once. Context matters: “You shouldn't steal” is a firm moral rule, while “You shouldn't wear stripes with plaid” is just fashion advice you might reasonably ignore.