English Grammar Guide

Modal Auxiliary Verbs

Modal Auxiliary Verbs - Can, could, will, would, should, may, might, must, ought and shall are modal verbs which 'help' other verbs to express a meaning.

Modal Verbs

The verbs can, could, will, would, should, may, might, must, ought and shall are verbs which 'help' other verbs to express a meaning. Modal verbs have no meaning unless they are used with another verb.

Modal Verbs - General principles

The auxiliary modal verbs 'would', 'may,' 'might', 'should', 'must', 'have to', 'need to', 'ought to', 'can', 'could', 'will', and 'shall' are invariable. Other than in the negative, their form never changes. Modal verbs NEVER change form; for example you can never add an "-ed" to a modal verb or an "-s" when referring to the third person singular. Modal verbs are NEVER used with other auxiliary verbs such as do, does, did etc. The negative is formed simply by adding "not" after the verb; questions are formed by inversion of the verb and subject. Modal verbs are auxiliaries, 'helping verbs' used with another verb (in the infinitive form) to modify its meaning. Modals can alter the meaning of the main verb in a number of ways:

Can and Could can be used to express possibility or ability:

  • I can swim.

  • Could you please help me with this?

May and Might can be used to express possibility or permission:

  • I may finish my paper tonight.

  • You may come with us, if you wish.

  • It might be helpful to have a map.

Must, have to, need to, ought to or should can be used to express obligation, moral obligation, necessity or to offer advice:

  • Students must hand in their homework on time.

  • You ought to see a doctor if you feel ill.

  • Children should never play with fire.

  • I have to buy some new shoes. These ones are getting very old.

  • You need to study harder or you will not pass your exams.

Should can also be used as a replacement for Must in formal notices such as:

  • All visitors should report to reception.  (This is more friendly than 'All visitors must report to reception'.)

Must can also indicate probability:

  • You must be exhausted!

  • He must play tennis pretty well.

Would can be used to express the conditional:

  • If I worked for your manager I would ask for a pay rise!

  • You wouldn't enjoy living in Scotland. It's very cold.

Will is used to express a future meaning (See future forms):

  • The train will arrive in an hour.


In affirmative sentences after a pronoun subject, the modals would and Will are often contracted:  

  • I would.... > I'd....    He would....  >  He'd.... (etc.)

  • They will....  >  They'll....    She will....  >  She'll.... (etc.)

Negative forms

The negative form is generally constructed by adding 'not'. In the negative, only the word 'not' can be contracted. 'May', 'Shall' and 'Ought to' are very rarely used in their negative forms, even in written English.


'Have to' and 'Need to' use a form of 'do not' before the modal to denote the negative.
'Will not' is contracted to 'Won't'
'Can not' can be contracted to 'Can't' but can also be written as 'Cannot'.

  • I would not do that if I were you.    >   I wouldn't do that if I were you.

  • You should not eat so much chocolate!   >   You shouldn't eat so much chocolate!

  • He does not have to go to work today.   >   He doesn't have to go to work today.

Question tags 

A question tag is a short question 'tagged' or added to the end of a statement which requires the listener to confirm or deny the statement. Modals can be used to ask a negative question after a positive statement. They can also be used to ask a positive question after a negative statement. The modal verb in the question tag is generally the same as the modal verb used in the initial statement and the subject pronoun is also repeated.

  • You would like to go to the cinema tonight, wouldn't you?

  • You can understand that, can't you?

  • You wouldn't want to work here, would you?

  • She won't be back, will she?