AUXILIARY VERBS are known as helping verbs because adding an auxiliary verb to a MAIN VERB helps the main verb convey additional information. The most common auxiliary verbs are forms of be, have, and do.
MODAL AUXILIARY VERBS are one type of auxiliary verb. They include:
Can, Could, May, Might, Must, Shall, Should, Ought to, Will, Would
They are Auxiliary verbs that provide additional and specific meaning to the main verb of the sentence.
How do we use modals?
e.g. Mary could play the piano
They do not accept conjugation
They do not need other auxiliary verbs
I will be able to work late next Monday. [Will be able is the future tense; will here is not a modal.]
Adding not between a modal and the MAIN VERB makes the CLAUSE negative:
We cannot work late tonight; I could not work late last night; I will not
be able to work late next Monday.
ALERT: You will often see negative forms of modals turned into
CONTRACTIONS: can’t, couldn’t, won’t, wouldn’t, and others. Because contractions
are considered informal usage by some instructors, avoid them in ACADEMIC WRITING.
- Conveying necessity
The modals must and have to convey a need to do something. Both must and have to are followed by the simple form of the main verb. In the present tense, have to changes form to agree with its subject.
e.g. You must leave before midnight.
She has to leave when I leave.
In the past tense, must is never used to express necessity. Instead, use had to.
How do I convey ability, necessity, advisability, possibility, and probability? 44a
PRESENT TENSE We must study today. We have to study today.
PAST TENSE We had to [not must] take a test yesterday.
The negative forms of must and have to also have different meanings. Must not conveys that something is forbidden; do not have to conveys that something is not necessary.
You must not sit there. [Sitting there is forbidden.]
You do not have to sit there. [Sitting there is not necessary.]
- Conveying advisability or the notion of a good idea
The modals should and ought to express the idea that doing the action of the main verb is advisable or is a good idea.
You should go to class tomorrow morning.
In the past tense, should and ought to convey regret or knowing something through hindsight. They mean that good advice was not taken.
You should have gone to class yesterday.
I ought to have called my sister yesterday.
The modal had better delivers the meaning of good advice or warning or threat. It does not change form for tense.
You had better see the doctor before your cough gets worse.
Need to is often used to express strong advice, too. Its past-tense form is needed to.
You need to take better care of yourself. You needed to listen.
- Conveying possibility
The modals may, might, and could can be used to convey an idea of possibility or likelihood.
We may become hungry before long.
We could eat lunch at the diner next door.
For the past-tense form, use may, might, and could, followed by have and the past participle of the main verb.
I could have studied French in high school, but I studied Spanish instead.
- Conveying probability
In addition to conveying the idea of necessity, the modal must can also convey probability or likelihood. It means that a well-informed guess is being made.
e.g. Marisa must be a talented actress. She has been chosen to play the lead role in the school play.
We use can to:
- talk about possibility and ability
- make requests
- ask for or give permission
e.g. can you speak any foreign language?
could: Past Possibility or Ability
Be able to
We use could especially with these verbs:
See hear smell taste feel remember
e.g. when we went into the house, we could smell burning.
We also use could to say that someone had the general ability to do something:
e.g. when tom was 16, he could run 100 meters in 11 seconds.
The past of could (do) is could have (done). We use could have to say that we had the ability or the opportunity to do something but did not do it.
e.g. we didn’t go out last night. We could have gone to the cinema but we decided to stay at home. (we had opportunity to go out but we didn’t)
- May & Might
Talking about things that can happen in certain situations
Saying that something was possible, but did not actually happen
Be in his office
I/you/he (etc.) may/might (not) Be having/waiting etc.
- Talking about the present, future or past with certainty
- Making a decision
- Making a semi-formal request
- Insistence; habitual behavior
- Making a promise or a threat
- Making offers
- Making suggestions
Polite requests and offers (a ‘softer’ form of will)
In conditionals, to indicate ‘distance from reality’: imagined, unreal, impossible situations
After ‘wish’, to show regret or irritation over someone (or something’s) refusal or insistence on doing something (present or future)
- Talking about past habits (similiar meaning to used to)
- Future in the past
- After ‘wish’, to show regret or irritation over someone (or something’s) refusal or insistence on doing something (present or future)
- Necessity and obligation
- Strong advice and invitations
- Saying you think something is certain
- Giving advice
Obligation: weak form of must
Things which didn’t or may/may not have happened
- Ought to
Ought to usually has the same meaning as should, particularly in affirmative statements in the present:
e.g. You should/ought to get your hair cut.
Should is much more common (and easier to say!), so if you’re not sure, use should.
- Used to
The auxiliary verb construction used to is used to express an action that took place in the past, perhaps customarily, but now that action no longer customarily takes place:
We used to take long vacation trips with the whole family.
Used to can also be used to convey the sense of being accustomed to or familiar with something:
e.g. The tire factory down the road really stinks, but we’re used to it by now
I like these old sneakers; I’m used to them.
Used to is best reserved for colloquial usage; it has no place in formal or academic text.
Source: google and English Grammar book