Full modal verb

A full modal (auxiliary) verb, is a verb (specifically a modal verb, and an auxiliary verb) used with a catenated verb to express possibility or necessity along with concepts such as obligation, permission, inevitability etc. They are used to express the speaker's opinion of a subject and thus to affect the modality of a sentence.

The full modal verbs in English are shown in the table below:

Present tense	Negative contraction(s)	Past tense	Negative contraction	Fundamental meaning (positive)	Meaning (negative)	Notes
can	cannot, can't	could	couldn't	Ability	Inability
will	won't, 'll not	would	wouldn't	Certainty	Negative certainty
may	?mayn't	might	mightn't	possibility	erm...	mayn't is rare.
shall	shan't	should	shouldn't	obligation	negative obligation
must	mustn't	must	mustn't	obligation	prohibition	Invariant preterite.

Grammatical properties

As a group the full modal verbs have a number of properties which separate them from lexical verbs and, to an extent, from primary auxiliary verbs.

• They can be negated with not. In other words we can say "will not" but we cannot say *"run not"—at least in modern English.
• They have no non-finite verb forms, i.e:
  • They have no infinitives, e.g. we can't say *"to can"; see infinitive of a modal verb.
  • They lack participles, e.g. -ing form, e.g. we can't say *musting.
• They often have weak forms, especially in the negative.
• They cannot co-occur in sentences. We cannot say, *"This may will happen".
• They can be used to form polar questions. (Yes/no questions).
• They can be used in ellipsis. In answer to the question "Can you swim?" the answer "Yes, I can" is understood to mean "Yes, I can swim."
• They are not conjugated or changed in any way. That means they have no third person "s", no past participle and no present participle/gerund "-ing" form.
• They are followed by the bare infinitive of another verb. That means they are not followed by a to infinitive.