Modals of Necessity, Prohibition, and Permission
Must, have to and have got to convey the idea that something is strongly required or obligatory, often by law. Must is a true modal, so its form never changes. Have to and have got to on the other hand, are phrasal modals which change forms to agree with their subjects. For example:
I must renew my passport before I go on vacation.
I have to/ have got to renew my passport before I go on vacation.
William has to/ has got to renew his passport before he goes on vacation.
Must not and cannot (and their contracted forms mustn’t and can’t) convey the idea that something is not allowed or prohibited, often by law. For example:
You cannot drive in Canada without a valid driver’s license.
Can is used to convey the idea that something is allowed at the time. Could is used to talk about something that was allowed in the past, and will be able is used to talk about something that will be permitted in the future. For example:
There’s plenty of room in the car. You can bring your friend with you.
Several years ago, people could smoke in almost all public buildings in British Columbia,
But now it is not allowed. For example:
In the future, people will be able to travel vast distances through space as easily as we travel around the world today.
Should conveys the idea that it would be wise to do something. In other words, it is a good idea to do it. For example:
In order to get to the airport in time to catch our flight, we have to leave home at 7:00 in the morning. Therefore, everyone should go to bed early tonight.
Do/ does are used with have to and has/ have are used with have got to when forming questions. Must is not commonly used in American English to form questions, nor is it used with do/does. For example:
It’s still early. Do you have to leave so soon?
She’s working hard. Has she got to finish the assignment tonight?
It’s still early. Must we leave now?