This article is additional to the main page dedicated to Modal Verbs.
There are several modal verbs that express obligation. Let's look at them according to their levels of obligation.
Must
The modal verb 'must' expresses the strongest obligation. We use it to say when something is restricted. It can be a rule, a regulation or a law. Also, restrictions can come from a person.
You should remember that the modal 'don't have to' is not a synonym to 'mustn't'.
Another important idea is that 'must have + past participle' doesn't express obligation in the past. This grammar construction expresses possibility. You can find more details on modals of deduction here.
For example 👇
A rise in deaths from preventable diseases must not be part of Covid-19's legacy. The Guardian
Contact-tracing apps must not be used for mass surveillance, warn experts. Financial Times
You must watch educational programmes instead of movies.
You must walk there carefully. The area is quite dangerous.
You must pay the taxes in time.
Have got to and have to
These modal verbs seem similar, but there is a slight difference between them. 'Have got to' sounds a bit stronger than 'have to'.
They both are used to speak about laws and rules. But you need to remember that they change their grammatical forms according to the context unlike other modal verbs.
I have to pay the tax.
She has to pay the tax.
Does she have to pay the tax?
I have got to work on Saturdays.
Have you got to work on Saturdays?
Should
This modal verb has the weakest level of obligation. We use it to give advice in common.
You should go to the doctor. You look very weak.
She should go to the GYM more often if she wants to lose weight.