Modal verbs of obligation, necessity and prohibition

Modal verbs in English are a special group of verbs. They different from the rest because they do not indicate any action or state and are not used independently. On the contrary, they reflect the modality of this action, the way the speaker relates to it.

In this article, we will consider modal verbs that express the meaning of duty, prohibition, and necessity. These include the following verbs: must, have to / have got to, need to, should, as well as negative forms mustn’t / can’t.


Must - conveys the meaning of obligation. It is often used when it comes to rules or laws that everyone must obey.

You must be careful when crossing the road.


Have to – expresses the meaning of necessity.

I have to finish reading the book this week.

The modal verb must is “stronger” and is usually used when the speaker decides what is necessary. However, we use have to when someone else makes a decision.

I must write the report today! (The speaker makes the decision.)

I have to write the report today. (The decision was made by someone else.)


Have got to - has a similar meaning with the verb have to, but is often used in everyday speech. Have got to is also relevant only in the present tense.

I’ve got to send them the letter.


I had to send them the letter last week.


Need - has the meaning of necessity, as well as have to. The verb need can be used both as a semantic verb and as a modal verb without changing its meaning. If we use it as a semantic verb, it is followed by to - infinitive, and in the form of a third person singular the ending -s is added. Questions and negative sentences are built with auxiliary verbs do / does.

I need to talk to the boss as soon as possible.

You don’t need to do this.


Should / ought to - convey the meaning of necessity, but they are much  "weaker" than must.

People should think / ought to think more about the environment.

Besides, we need verbs should/ought to in order to give a piece of advice.

You should call your friend.


Mustn’t / can’t - help us express a prohibition and say that something is against the rules or the law.

You mustn’t smoke in the library.

You can’t park here.


We said above that the modal verbs must and have to are close in meaning. However, when they are used in a negative form, their meaning changes dramatically.

If Mustn’t shows the prohibition, don’t have to / don’t need to indicate no need or lack of necessity.

You don’t have to / don’t need to work on Saturday. (No need, you can do it, but what for😉)

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