Magnificent Modals! Part 1: What are modal verbs? Tips on modals of obligation

Image courtesy of Sabine Sauermaul

Some of the questions we’re going to explore this week include:

  • What are modal verbs?
  • What do modal verbs do?
  • Some tips and explanations on modal verbs of obligation.

What are modal verbs? 

There are many modal verbs. They include can, could, would, should, must, will, may, might and shall. There are also some semi-modals which have some of the characteristics of modal verbs but not all of them. Semi-modals include have to, ought to, dare, had better and need.

The main characteristics of modals verbs are:

  • Modal verbs are auxiliary verbs. This means that they do not need an additional auxiliary to make negatives or questions, e.g. the question form of Simon can swim is Can Simon swim? not Does Simon can swim?
  • Modal verbs also do not change their form or spelling unlike other verbs, e.g. Simon can swim not Simon cans swim.
  • Modal verbs have no infinitive and no present participle (-ing form) or past participle.
  • Modal verbs are generally followed by one of the infinitives in English, without “to”. The eight different possibilities of how we can use modals are below (we will look at these in more details later).
      1. modal verb + base form e.g. will study
      2. modal verb + be + present participle e.g. will be studying
      3. modal verb + have + past participle e.g. will have studied
      4. modal verb + be + past participle e.g. will be studied
      5. modal verb + have + been + present participle e.g. will have been studying
      6. modal verb + have + been + past participle e.g. will have been studied
      7. modal + be + being + past participle e.g. will be being studied (NOT IN COMMON USE)
      8. modal verb + have + been + being + past participle e.g. will have been being studied (NOT IN COMMON USE)
Photo courtesy of hvaldez1

Let’s take a sentence and see how modal verbs can change it. If I use a sentence with no modal verb, for instance, “He’s studying to be an engineer” then I am giving the information in the sentence as a fact – I am making no comment on it. However, I can use modal verbs to express other information. Think about the differences between:

  • He has to study to be an engineer (maybe he’d prefer to study to be a lawyer)
  • He must study to be an engineer (he can’t be an engineer without studying)
  • He should study to be an engineer (this is what I think is best for him to do in the future)
  • He won’t study to be an engineer (this is something that I predict will not happen in the future or that he doesn’t want to do)
  • He should have studied to be an engineer (he didn’t do this in the past but it would have been a good idea)
  • He could study to be an engineer (this is one possibility but he might choose another career)

There are many more examples, but from this short list we can see that modal verbs add a lot more information to a sentence.

boys on jetty

So, which modal verb do we use and when?

In general, we divide modal verbs into categories according to what they do to the verb and the extra information that gives. So there are modals of:

  • possibility
  • probability
  • necessity
  • obligation
  • permissibility
  • ability
  • desire
  • contingency


In this post, we’re going to look at obligation. There are two modals of obligation, should and must and two semi-modals, have to and ought to. Let’s look at should first.


We use should for fairly mild obligation, generally when we mean “I think this is a good thing to do”. Its opposite is shouldn’t which we use to mean “I think this is not a good thing to do” You shouldn’t eat sweets tells the listener that the speaker thinks eating sweets is not a good thing and s/he is giving advice not to eat them.

When we use should to talk about the past, we use should + have + past participle. This is used to talk about something that was a bad thing to have done in the past. For example, You should have gone to the doctor means that it was a good idea in the past but you didn’t do it. We often use this structure to talk about regrets, e.g. I shouldn’t have eaten so much cake – I feel sick! A chain of opticians has used this structure in its advertising, and here is a link to a funny clip

We can also use should to express a relationship something between recommendation and obligation. We can use be supposed to (which can be used in the past or in negative forms) to express this idea , e.g. We should bring books back to the library before they are due / We are supposed to bring books back to the library before they are due. 

(We need to bear in mind that should performs other functions in English, but we’re just looking at the function of obligation here and we’ll look at it in more detail in future posts. )


Ought to is a semi modal because it doesn’t follow the rule about no “to” after a modal. The meaning of ought to is almost exactly the same as should, so You should read this means the same as You ought to read this. The negative is ought not to or oughtn’t to. However, Irish people very rarely use any form of ought – it is much more commonly used by Americans.


The other modal of obligation is must. This is stronger than should and generally means that the speaker thinks something is necessary; or it can be a strong recommendation, e.g. The movie is great, you must see it! The “source” of the obligation with must is the speaker, so it can be very strong if you are using it. Native speakers often avoid using it – must is more often used in signs than in spoken English.

The opposite of must  is mustn’t and this means there is a negative obligation (or prohibition). You mustn’t open the present before your birthday means there is an obligation NOT to open the present before your birthday.  Mustn’t is used to prohibit actions (usually actions in the near future), but it sounds very harsh in English and again native speakers often prefer to avoid it and use softer modal verbs such as shouldn’t. 


As we have mentioned, native speakers often prefer to avoid both must  and mustn’t and prefer to use should or have to instead – let’s look at have to now.


Have to is one of the two semi-modal verbs which we use to talk about obligation. It is not a proper modal verb because is doesn’t follow all the characteristics of modal verbs we talked about at the beginning of this post. Unlike other modals, it changes form. This means that we do have:

  • the third person -s (He has to go to work at 7 a.m.)
  • we use auxiliary verbs to make the negative and question forms (Does he have to work at 7 a.m.? He doesn’t have to work at 6 a.m.)
  • an infinitive (to have to), past participle (have had to)and present participle form (having to)


In the positive, have to has a similar meaning to must in that it expresses fairly strong obligation. If I say He must go to work at 7 a.m. this means more or less the same as He must go to work at 7 a.m. However, the first sentence is stronger and focuses on the obligation itself rather than on the situation which creates the obligation. This is because when we use have to, the “source” of the obligation is not the speaker but the rules or the general situation. This makes it less direct, and therefore softer than must. If you’re not sure whether to use must or have to, it is often best to choose have to for this reason.

The past of have to is had to. Because there is no past of must, we can use had to for the past of both must and have to. E.g. I must go to the doctor tomorrow changes to I had to go to the doctor yesterday in the past; and I have to go to the doctor tomorrow also changes to I had to go to the doctor yesterday. The past of don’t have to / doesn’t have to is didn’t have to.

BUT (and it’s a big “but”!)

In the negative, don’t have to / doesn’t have to / didn’t have to etc. are very different from mustn’t. They express a lack of obligation; or no necessity. So, She doesn’t have to go to work at 7 a.m. means that there is no necessity for her to do this, she can stay home if she wants. However, She mustn’t go to work at 7 a.m. means there is a prohibition on this – perhaps the night guard is still on duty at 7 a.m.



We have covered a lot of ground in this post – now it’s your turn to check out some of the links we have found to help you with modal verbs:

These links are to help you practice and also have lots more information on modal verbs:


YouTube lesson for lower levels about modals of obligation 

Linguapress logo Linguapress also has some information and exercises at

English page has some details of must at and of should at

Please let us know if you have any comments on this blog (or any of our blog posts!) We’ll be back soon with more “Magnificent Modals”!


One Comment Add yours

  1. Phat Ton says:

    it is very good for students to do research, but it is not detail yet. About type of obligation.

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