Modal Verbs for Obligation & Prohibition

This lesson about modal verbs for obligation and prohibition is part of an intermediate English course.

We often use modal verbs to talk about rules. Sometimes to talk about obligations, sometimes prohibition and sometimes recommendations. In this lesson we will review what modal verbs are, what some common modal verbs mean and how to use each of them to talk about rules, obligations, prohibition and recommendations.

What are Modal Verbs?

A specific type of verb that has no conjugation!

For example: “Can”.

  • I can, you can, he can, we can, they can, 

It’s all the same.

A verb that is followed by “the bare infinitive (without “to”)”

  • “I can swim.”, “He can sing.” etc.. 


We use must for;


  • “You must wear a seatbelt when you’re driving.”


  • “I must remember to call my mom.”

strong recommendations;

  • You must see this film, you’ll love it!

Have to

Have to is also used for rules.

“He has to wear a uniform”

The negative is with the auxiliary “do”. It means that there is no obligation to do something.

“We don’t have to work tomorrow”

A note about “Have to”.

This verb is a “semi-modal verb” we do have to conjugate it for different persons, for example

He has to work.

However, the following verb is still used in the bare infinitive / base form.

Don’t need to / Needn’t

A synonym of “Don’t have to” is “Don’t need to”, which can be expressed as “needn’t”.

“We don’t have to work.” 

“We don’t need to work.” 

“We needn’t work.” 

Note that we don’t put “to” after “needn’t”, it’s a modal verb.

The Difference between Must & Have to

 Internal / ExternalSpecific / General
Have toExternalGeneral

The Difference between Mustn’t & Don’t Have to

Although the positive versions are synonymous, the negative versions are 


  • Don’t have to  means No Obligation, You can do it if you like, but there isn’t a necessity. It isn’t like in Castellano!
  • Mustn’t means that it is Prohibited.

Should / shouldn’t

We use should for recommendations;

  • “You should sleep for at least 7 hours.”
  • “You should hear this music, you’ll like it.”

The negative form is just a negative recommendation;

  • “I shouldn’t eat so much cake!”
  • “You shouldn’t drink milk that smells bad.”

Ought to / Ought not to

“Ought to” is synonymous with “Should”

It is more formal and less common than should, but works exactly the same.

  • “You ought to sleep for at least 7 hours.”
  • “You ought to hear this music, you’ll like it.”

The negative form is just a negative recommendation;

  • “I ought not to eat so much cake!”
  • “You oughtn’t to drink milk that smells bad.”

Had Better

Had better is a synonym of should.

We had better go.

He had better study if he wants to pass the exam.

Can / Be Able to

We can use can and be able to to talk about abilities and possibilities. Can and Be Able to are synonymous, however, Be Able to is more formal (and a lot more versatile, it exists in all tenses, can only exists in past, conditional, present and present with future meaning).

I like being able to walk around the city.

this means that I have the the possibility or the ability to walk around the city.

She isn’t able to come to the phone right now.

She can’t come to the phone right now.

May / Might

We can use the words may and might to talk about possibility & permission.

We can use might for possibility.

It might rain.

It might not be available.

However, we can’t use might for permission. Only may has this meaning.

You may sit down.

You may use the phone.

PermissionMay I use the phone?N/A
PossibilityIt may rain later.It might rain later.


Check that you have understood this topic with these interactive grammar exercises.