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Special or Modal auxiliary verbs

This is a complicated area but this is a simple grammar.  We will not cover all of these verbs and their meanings.  The following looks at the most common and simple modal auxiliary verbs only.

feelings

Modal auxiliary verbs express how we feel about something

They express 5 main ideas and there are 6 common ones.

The 5 ideas modal auxiliary verbs express are:

  1. possibility and logical certainty: Do you think it will happen or has happened?  How sure are you?
  2. willingness and promises: Do you want to do something?
  3. ability: Are you able to do something?
  4. obligation, permission and prohibition: Are you forced to do something?  Are you allowed to do something?  Are you forbidden to do something?
  5. suggestions and advice: Is it a good idea to do something?

The six modal auxiliary verbs in this grammar are:

There are some more modal auxiliary verbs but these are the most important.

special

Why are these verbs special?
  1. The grammar is different.
    1. Most verbs make a question by adding do, did or does (for example: Did you see him?, Does he live here? etc.).  These verbs do not.  With modal auxiliary verbs we just put the verb before the subject.  For example:
      You can see it → Can you see it?
      I must go → Must you go?
      etc.
    2. Most verbs take an -s at the end with he, she and it in the present tense (for example: He goes, It rains, She smokes).  These verbs do not:
      I can, he must, they can't, she ought to, everybody should
    3. Most verbs make a negative with do, did or does (for example: I don't understand, She didn't arrive).  These verbs do not.  With modal auxiliary verbs we just put not after the verb.  For example:
      I must not
      She should not
      We could not
  2. The meaning of these verbs can only be seen when they are with a main verb.  For example, we can understand
    They smoke
    She arrived
    She must come
    You can't help
    etc.
    but we cannot understand
    They could
    She must
    We can
explained

Some common modal auxiliary verbs explained

read

can / could / be able to

He can read English well.

This verb is used for:

  1. Ability:
    He can read German but can't speak it well
    He could play the piano well as a child
    He was able to speak Italian when he was 6
  2. Permission (present and future):
    Can I come in?
    No, you can't
    Could I talk to you tomorrow?
    be able to     is not used for permission
  3. Possibility and impossibility:
    Nobody can be sure
    The train could be late
    She can't be so silly!
    be able to     is not used to talk about possibility
may

may / might

You may not park here!

This verb is used for:

  1. Permission:
    You may ask a question now
    May I smoke here?
    You may not leave before 6
  2. Possibility and impossibility (present and future):
    We may arrive a little late
    He might come early
    Might he be late?
    I might not arrive on time
dogs

shall / should

Shall we take the dogs for a walk?

This verb is used for:

  1. To make questions:
    Shall I do my homework now?
  2. To make suggestions:
    Shall we go?

Should is much more common and can express:

  1. Obligation:
    You should write to your mother more often
  2. Logical certainty:
    He should be there by now
    Notice that the negative of He should be there by now is He can't be there yet.
  3. Advice:
    You should take something for your cold
would

will / would

Would you like milk?

This verb is used for:

  1. Willingness:
    I'll get the milk
    Will you have another biscuit?
  2. Offers (would only):
    Would you like some cake?
    Would you enjoy a little music?
  3. Intention or a promise:
    I'll send you an email soon
    He told me he would write soon
  4. Possibility and certainty:
    It'll probably rain soon; it often does in November
    He knew it would rain
    He believed I would come
must

must / have to

You must not say that!

This verb is used for: 

  1. Obligation:
    You mustn't speak to me like that
    You must be home at 6
  2. Logical necessity / deduction:
    That must be his father
    There must be an error in the data

Making the negative of must is not easy.  For example, with
    You must take the medicine every day
The negative can be

  1. You mustn't take the medicine every day (i.e., you are obliged not to)
    or
  2. You don't have to take the medicine every day (but you can if you want to)

and with:
    The figure must be correct
The negative can be

  1. The figure mustn't be correct (i.e., you must give the wrong figure)
    or
  2. The figure can't be correct (i.e., there is clearly a mistake here)
ought

ought to

She ought to be in bed.

This verb is used for: 

  1. Obligation (weaker than must):
    She ought to ask if she doesn't know
  2. Logical deduction / expectation:
    The bus ought to be here by now
time

Time and tense

Only some modal auxiliary verbs have past-tense forms.  Here's a list.

Present Past
can could
may could / might
shall should
will would
must had to
ought to ----

The past of may

may has two past forms.  One for possibility, one for permission.  We can say
I could ask questions (permission: I was allowed to ask questions)
and
I might ask questions (possibility: it is possible that I will ask questions)

The past of must

is usually had to.
I must go tomorrow → I had to go yesterday
I must do it now → I had to do it then

Click here to take a short test on modal auxiliary verbs and their meanings.