Modal auxiliary verbs

Modals are a very complex area of English grammar, so in this quick guide we will not be able to go into much detail, but we will at least get an overall idea of what their function is in a sentence. In an earlier section of this guide we looked at how the verb phrase can be broken down into its constituent parts and we noted that one of these parts was called a modal auxiliary verb. Just to remind you of the previous examples, a section of the chart has been reproduced below:

auxiliary verb
auxiliary verb(s)
Main verbObject
Youshouldhave beenwatchingthe baby

Before we look at some of the possible meanings of modal auxiliary verbs we need to have some idea of what constitutes a modal in English and where they occur in a sentence. A few more examples should enable us to answer the second of these points fairly quickly and easily – the modals are in bold:

  • He should be here by now.
  • could swim quite well when I was younger.
  • You mustn’t blame yourself for this.
  • You might have discussed it with me first.
  • You can’t be serious!
  • Could you open the window please?
  • Must you make so much noise?
  • She had to take her brother along with her.
  • We ought to be going.

It should be clear from these examples that the modal verb occupies the first position in verb phrase, coming before any other auxiliary verb (like have or be) and the main lexical verb.

In questions the modal verb is simply inverted with the subject of the sentence as in examples 6 and 7 and it also carries the negative particle not (examples 3 and 5).

The subject of the sentence has no effect on the form of the modal since almost in all cases they do not change at all.

So, a modal verb is quite simple as far as its form and position in various types of sentence are concerned; but what exactly are the modal verbs in English? The chart below lists the main modal auxiliaries that you are likely to meet and divides them into two categories pure modals and semi-modals, although in most cases the distinction is merely formal and their meanings are not affected by this division.

Pure modalsSemi-modals
canought to
couldhas/have (got) to
maybe able to
need ***

*** need is a special verb since as an auxiliary it is almost always negative and it is also a lexical verb as in sentences like he needs to speak to you now, while it acts as a modal verb in sentences such as you needn’t come to work tomorrow where it has the same meaning as don’t have to.

The forms of pure modals

The main characteristics of the pure modals are:

  • they never change their form irrespective of the subject of the sentence
    e.g. he can swim, not *he cans swim
  • following on from the above feature, they do not change to show past tense
    e.g. she had to leave not *she musted leave
  • they all carry the negative of the sentence by the addition of not/n’t
    e.g. I can’t remember not *I don’t can remember
  • they all form questions by inversion with the subject of the sentence.
    e.g. should I stay?
  • they are all followed by the base form of the verb without the addition of to
    e.g. he can swim not *he can to swim
The forms of semi-modals

You will notice that this type of modal is made up of two or more separate words, the last one invariably being to. They are all modal in meaning but not in form as they behave differently in a sentence from the pure modals. It is perhaps best to think of the semi-modals in the form with the to infinitive that is given in the table rather than thinking of them as modals that need to + base form. We need to look at the form of each individual semi-modal separately.

Be able to

We use this semi-modal to express possibility or the ability to do something, but unlike the pure modals, be able to has a full range of tenses and also needs to inflect to show agreement with its subject. For example:

  • He is able to offer you the best price possible.
  • We were able to get in to see the film.
  • They haven’t been able to find the missing document.
  • So, you aren’t able to help.

Notice that the negative is carried either by the be element or the auxiliary verb that is closest to the subject of the sentence. It can also be accompanied by any of the pure modals:

  • I will be able to see you after lunch.
  • They might not be able to put us up for the night.

Has/have (got) to

This is used to express necessity or obligation to do something and shares some of the features of be able to discussed above. The have element of the form has to change to agree with its subject. Although it is normally used in the present tense, it also has its own past (had to) and can be used with pure modals to show the future or the attitude of the speaker:

  • They have to be more punctual.
  • He has to take responsibility for the accident.
  • I had to help my father repair his car.
  • We will have to put this off until tomorrow.
  • You shouldn’t have to suffer in silence.
  • You don’t have to come if you don’t want to.
  • He didn’t have to do all the shopping.

From these few examples it should be clear that the negative not again attaches itself to the auxiliary verb (modal or main) that comes immediately after the subject of the sentence.

Ought to

It is usually claimed that the meaning of ought to is the same as should whether it refers to giving advice or making a logical deduction. So, to most native speakers the following sentences with ought to and should feel the same:

  • You ought to see a doctor.
  • You should see a doctor.
  • They ought to have got back home by now.
  • They should have got back home by now.

In practice, most speakers tend to prefer should for negatives and questions because the ought to and oughtn’t … to forms can sound rather clumsy and awkward.

  • Ought you to be doing that?
  • They oughtn’t to (ought not to) do that.
  • Oughtn’t we to leave now?
Meanings of modal verbs

The main function of modal verbs is to allow the speaker or writer to express their opinion of, or their attitude to, a proposition. These attitudes can cover a wide range of possibilities including obligation, asking for and giving permission, disapproval, advising, logical deduction, ability, possibility, necessity, absence of necessity and so on. The problem with each modal verb is that it can have more that one meaning and the interpretation of a particular modal will depend heavily on the context in which it is being used. The following examples should help to illustrate this point.

  • It might take more than a week. (possibility)
  • You might have told me about it! (showing disapproval)
  • He must take his medicine three times a day. (obligation)
  • He must be French. (logical deduction)
  • I can’t lift that suitcase by myself. (ability)
  • That can’t be the right answer. (logical deduction)
  • May I look at the questions now? (asking for permission)
  • They say it may snow tomorrow. (possibility)

You probably also noticed from the examples that notions like permission and possibility can be expressed using different modal verbs – this, of course, only serves to complicate matters further since one modal verb can have more that one meaning, and one meaning can be expressed by more than one modal verb. In the space that we have available here it would be impossible to cover all the meanings of each of the modals, so as examples we will look at some of the ways that obligation and logical deduction can be expressed.


The two main modals here are must and have to. The difference between them is usually given as follows: must is used to express an internal obligation that is imposed by the speaker, while have to refers to rules and regulations that are imposed from outside the speaker. Again, as with many points of grammar this is only intended as a rough guide.

To express a lack of obligation we cannot just automatically add not to the modal verbs without thinking more carefully about it first. How do you feel about the following sentences for instance?

  • He must sing loudly.
  • He mustn’t sing loudly.

In the first sentence you would probably agree that this is obligation originating from, say, a teacher or someone with authority. The second sentence, however, does not express a lack of obligation but a prohibition to do something. The form that we use to express a lack of obligation could be one of the following:

  • He doesn’t have to get up early.
  • He doesn’t need to get up early.

This lack of balance in the use of modals can cause many problems for people who are learning English since it is quite illogical.

Logical deduction

This is another area of modal use that is fraught with difficulties for reasons similar to those just discussed above. Look at the following sentences:

  • The telephone rings:
  • That’ll be Frank.
  • That must be Frank.
  • That should be Frank.
  • That could be Frank.
  • That might be Frank.
  • That may be Frank.

The modal verbs used here have been listed in what many consider to be the order of likelihood of something being true. You may or may not agree with this listing, but it gives you some idea of some of the choices available for drawing logical conclusions from situations. If we look at the negatives of these sentences, however, you can see just how much more complex it can become:

  • That won’t be Frank.
  • That mustn’t be Frank.
  • That shouldn’t be Frank.
  • That couldn’t be Frank.
  • That mightn’t be Frank.

Many of these sentences now denote completely different attitudes to the situation and you may even agree that some of them are either not English or are only marginally acceptable. The sentence which has probably moved furthest from its original intention is number 2 which sounds very odd. In fact, the negative of must when we talking about deduction is can’t – one more example of how complicated and counter-intuitive the system of English modals can be.

Past time with modals

We noted earlier that the pure modals do not change to show tense. Most of these modals do in fact have either present or future reference, but sometimes we need to refer back to the past. With the semi-modals there is little problem, but how can we do this for pure modal verbs? You may have picked up from some of the previous examples that one way to do this is to insert have immediately after the pure modal. But this is not always the case since can has its own past tense could when it refers to general ability. Some examples should help:

  • I can speak German.
  • I could speak German when I was seven years old.
  • You should see this film.
  • You should have seen this film.
  • Indonesia must be hot.
  • Indonesia must have been hot.
  • He could find his wallet.
  • He could have found his wallet.

Notice that in the third pair of sentences the meaning of must is logical deduction not obligation. If we want to use must for obligation then the past tense is had to.

  • She must visit her mother.
  • She had to visit her mother.