Modals- Should/shouldn't/must/mustn't

Used to give advice or recommendations

Considered as mild obligation or advice

Example A: You should save some money.

Example B: You shouldn't wake up late in the morning.

affirmative: You should tell someone about the problem.

negative: You shouldn't forward nasty messages.

question: Should you reply?

short answer: Yes, you should./No. you shouldn't.

affirmative: They must stop fighting.

negative: You mustn't deal with the problem alone.

question: Must I tell my friends?

short answer: Yes, you must./No, you mustn't.

Used to express strong obligation (possibly based on the speaker's opinion) ;-)

Examples:

For mothers- You must study. You mustn't sleep all day long.

For teens: I must finish playing this level. You mustn't be concerned- exams are next year!