Modal auxiliary verbs

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Modal auxiliary verbs

Modal auxiliary verbs

These are verbs expressing needs, necessities, possibilities, desires or intentions. They always have 1 invariable form, which does not depend in any way on the tense, the subject or the action in the sentence. The main modal auxiliary verbs include:

  • Can /Could
  • May / Might
  • Must / Have to
  • Shall / Should
  • Will / Would
  • Ought

and their negative forms. They also often replace the primary auxiliary verbs:  To have, To do and To be.

We have also prepared a separate set of modal auxiliary verb examples for the article below.

They always have the same form

All modal auxiliary verbs are always used in their basic form. We never conjugate them or adapt them to tenses or subjects. The only exception to this is the formation of the negative forms of these verbs using the negative “not“.

If the speaker wants to express his idea using modal auxiliary verbs in the past tense, for example, he is forced to use the form of the modal verb that applies to the past tense.

For example:
  • I could have done this.

This is also why you never encounter the infinitive (to can/to must), present participle (canning/musting) or past participle (canned/musted) of modal verbs. These forms simply do not exist in the English language.

The main reasons for using modal verbs include:

Expression possibilities

  • We could tell her to come with us.
  • We might go there sooner.

Expression of need

  • I must focus on this problem.
  • We should go there.
  • I ought to move to a big city because of my job.

Sentence construction in modal verbs

An auxiliary modal verb is always linked to the main verb in its basic form. The past or future tenses are expressed by the modal verbs chosen directly – so there is no need to modify the main verb.

Subject + modal verb + main verb in its basic form + the rest of the sentence.


  • I can do this!
  • She shall live in the USA with me.
  • You could bake a tart for your mum.
  • I would like to live a happy life.
  • He must stay here with her!

Wrong phrases

Never use modal verbs with the form “of“, such as “could of, should of, would of“.  This is a mistake. The correct form in this case is “could have (could’ve)“, “should have (should’ve)” or “would have (would’ve)“.

Can’t x cannot x can not

The verb “can’t” is commonly used to express the negative meaning of a sentence. However, it is a slang form that can be expanded into 2 other forms – “cannot”, “can not”.

What is the difference between them?

  1. Can’t – this form is informal and is used in common speech.
  2. Cannot – this form is an expansion of the verb “can’t” and is a more polite way of expressing disapproval. This form is not aggressive and is used in normal speech.
  3. Can not – this form is used when you want to express a really strong disagreement.

Might x may

These 2 modal seemingly similar verbs differ in meaning.


The verb “might” is used to express uncertainty or to form an interrogative sentence. It is a verb with a strong meaning that is very formal.

Very often we can see this form when forming polite questions or when acting in an assertive or even slightly aggressive manner. “Might” is also used to express hypothetical or not very likely scenarios.

  • I am worried she might be late.
  • Might see you tomorrow!
  • We might finish it faster together, mightn’t we?


We use the verb “may” when we are not sure about future events. It can also be used to express questions, uncertainties or factual and probable events.

Compared to the verb “might“, “may” sounds more casual and less harsh – it has a more empathetic and pleasant feel.

  • May I ask you something?
  • My friend may be joining us tomorrow.

However, it can also be used to emphasize disagreement.

  • You may not do whatever you want until you’re an adult!

Would x Should x Could

These three modal verbs are very similar, but they have completely different meanings.


The verb „would“ is formed from the auxiliary verb “will, which helps form the future tense. It is often used to express conditional sentences by combining the verbs “will” and “would“.

  • If I was you I wouldn’t do it.
  • I would call him if I had his number.


The verb “could” comes from the auxiliary verb “can and expresses possibility.

  • They could have told me.
  • Could you please borrow me some money?


This verb comes from the nowadays not very common form shall. It is a modal auxiliary verb expressing recommendation or advice.

  • Maybe we should tell her the truth.
  • Should we do it?

Ought to

The verb „ought“ is an exception to forming sentences. It is used to express advice or recommendation.  While other modal auxiliary verbs are always followed by the verb in the base form, this is not the case with „ought“. It is always associated with the preposition “to” and the infinitive verb.

Subject + ought + to + infinitive + the rest of the sentence.


  • I ought to clean my room because my mum is coming soon.
  • You oughtn’t to be there.

TIP: See 10 examples of sentences using modal verbs.

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