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.
A specific type of verb that has no conjugation!
For example: “Can”.
It’s all the same.
A verb that is followed by “the bare infinitive (without “to”)”
We use must for;
Rules;
Obligations;
strong recommendations;
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”
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.
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.
Internal / External | Specific / General | |
Must | Internal | Specific |
Have to | External | General |
Although the positive versions are synonymous, the negative versions are
TOTALLY DIFFERENT!
We use should for recommendations;
The negative form is just a negative recommendation;
“Ought to” is synonymous with “Should”
It is more formal and less common than should, but works exactly the same.
The negative form is just a negative recommendation;
Had better is a synonym of should.
We had better go.
He had better study if he wants to pass the exam.
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.
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.
May | Might | |
Permission | May I use the phone? | N/A |
Possibility | It may rain later. | It might rain later. |
Check that you have understood this topic with these interactive grammar exercises.