The Core Modals
The core modals (modal verbs/modal auxiliaries) are a closed set (i.e. new verbs are never added to the set) of verbs that convey the speaker's attitude to the likelihood of the event/state denoted by the following verb, or to express such concepts as ability, certainty, etc. They differ formally from other verbs in a number of ways:
1. They do not use DO in their negative, interrogative or emphatic forms :
I go --- I do not go --- do I go? --- I do go.
I must ... --- I must not ... --- must I ...? --- I must ....
2. They do not use DO as 'code':
he goes, doesn't he? --- does he go? yes, he does. --- he goes, and so do I.
he can ... can't he? --- can he ...? yes, he can. --- he can ..., and so can I.
3. They do not take –(e)s in the 3rd person singular form: she goes --- *she musts ...
4. They have no non-finite forms (infinitive, participle or gerund):
to go --- going --- gone. .
*(to) may --- *maying --- *mayed
5. They cannot co-occur with themselves or with other modals:
they can go -- they will go ... ---
*they will can... --- *they can will...
6. All of the modals except (in most dialects) MAY have a shortened negative form with n't:
can't --- couldn't --- mightn't --- mustn't --- shan't --- shouldn't --- won't --- wouldn't
The only verbs that have all these properties are can, could, may, might, must, shall, should, will and would, the core modals. Whether or not the italicised forms are the past-tense forms of the forms preceding them or are different modals will be considered later.
As the three dots following the modals in the examples above imply, modals cannot stand alone as verbs – they must collocate with another verb, though the other verb may be implied, as in:
May I? (some unexpressed verb such as sit here or smoke will be clear from the context of situation)
Need can operate as a full verb followed by a noun or to-infinitive, or as a modal:
does he need money/to go? he doesn't need money/to go. he needs money/to go. need he go? he needn't go. *he need go
However, modal NEED is used mainly in (near- ) negative, occasionally in interrogative and very rarely in affirmative constructions. Because of these restrictions on its use as a modal, and because it can operate as a full verb with little practical difference in meaning, it is perhaps less confusing for learners if NEED is introduced at a different time from the nine core modals.
Dare can also operate as a full verb followed by a to-infinitive, or as a modal:
does he dare to ask her? he doesn't dare to ask her. dare he ask her? he daren't ask her.
However, modal DARE is not common. Learners are better told that DARE normally operates as a full verb. Such expressions as I dare say can be taught as lexical chunks.
Ought (usually referred to as OUGHT TO) is a slight problem. Unlike the core modals, it normally requires to before a following infinitive. Also, many speakers of English form its interrogative and negative with the past tense of DO.:
did he ought to go? He didn't ought to do that. (some speakers regard these forms as sub-standard) Ought he do go? He oughtn't (to) do that.
A further complication for students is that whenever OUGHT is used, it can be replaced by SHOULD with virtually identical meaning. However, the reverse is not always true:
You ought to get up earlier. You should get up earlier. If I should die, think only this of me.. *If I ought to die, think only this of me..
Life is made simpler for students if they learn to understand OUGHT but always use SHOULD.
Used to has some of the characteristics of modals, but is so different in other ways that it is best considered separately.
IS TO , HAVE TO, HAD BETTER, WOULD RATHER, and even BE ABLE TO and BE SUPPOSED TO have some of the characteristics of modals, often convey very similar meanings to modals, and sometimes need to be used where appropriate modal forms do not exist. Some grammarians find it useful to use such terms as quasi-modals or near-modals for these and other verbs and verb phrases. However, they are not modals.
Past Tense Forms
In back-shifting, COULD, MIGHT, SHOULD and WOULD function as the past-tense form of CAN, MAY. SHALL and WILL :
"Can you swim?" --- John asked me if I could swim.
"Shall I make the tea?" --- I asked Mary if I should make the tea
However, in other contexts, they do not function as past-tense forms, for which substitutes forms need to be used; for example:
I can see him this afternoon. → *I could see him yesterday afternoon.
I was able to see him yesterday afternoon.
In addition, the ‘past-tense’ forms can refer to the same time points/periods as their ‘present-tense’ equivalents:
I can see him this afternoon. --- → I could see him this afternoon,
This explains why some teachers prefer to present COULD, MIGHT, SHOULD and WOULD as separate modals rather than as the past-tense forms of CAN, MAY. SHALL and WILL. This is a question for individual teachers to deal with in ways that seem most useful for their students.