Modals of obligation: must / have to / should
In English we use "must", "have to", and "should" to talk about obligation and advice: "must" and "have to" express strong necessity or rules, while "should" gives advice or recommends what is a good idea.
Must and have to: strong obligation
We use "must" and "have to" to say that something is necessary or required. "Must" often shows the speaker's own feeling or a strong personal rule, while "have to" usually shows an outside rule, a law, or a fact. "Must" is followed by the base verb with no "to" (you must go), but "have to" changes form for the subject (he has to go).
- •You must wear a seatbelt in the car.
- •I have to finish this report before Friday.
- •She has to wake up early for work.
- •We must be quiet in the library.
Should: advice and recommendations
We use "should" to give advice or to say what is a good idea, not what is strictly necessary. It is weaker than "must" and "have to", so it suggests rather than forces. Like "must", "should" is followed by the base verb with no "to".
- •You should drink more water.
- •He should see a doctor about that cough.
- •We should leave now if we want to be on time.
- •You should not eat so much sugar.
Negatives: a big difference in meaning
"Must not" (mustn't) means something is prohibited or not allowed. "Don't have to" means there is no obligation, so you are free to choose. "Shouldn't" means it is not a good idea, but it is still your choice.
- •You mustn't smoke in here. (It is forbidden.)
- •You don't have to come if you are tired. (It is optional.)
- •You shouldn't drive when you are sleepy.
- •Students mustn't use phones during the exam.