Modal verbs and grammatical mood

Modal verbs help express possibility, obligation, expectation and intention. The same auxiliary can serve several purposes, so its meaning depends on the statement and context in which it appears.

Mood and communicative purpose

Grammatical modality attaches belief, attitude or obligation to a statement. ¹ Word order, auxiliary verbs, adverbs such as “perhaps”, and constructions such as “It is unlikely that…” can help express it. ¹ Five distinctions provide a starting point for identifying a sentence's purpose. ^{1, 2}

Label	Purpose
Indicative	Statements presented as fact or reality.
Interrogative	Questions.
Imperative	Commands and requests.
Conditional	A state that would cause something else to happen.
Subjunctive	A hypothetical or contrary-to-reality state, such as a wish, desire or imagined situation.

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The term realis concerns statements of fact and reality, while irrealis concerns other kinds of meaning. ^{3, 2} Modality includes the following distinctions. ²

Modality	Purpose
Epistemic	Doubts, certainties and guesses.
Deontic	How things ought to be.
Commissive	A commitment to act, such as a promise or threat.
Directive	Commands and requests.
Volitive	Wishes and desires, as in “If only…!”

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The distinction between how doctors practise and how they ought to practise is a distinction between an indicative description and deontic modality. ⁴ An expression of possibility or desire can soften a command: “Could I have…?” addressed to a waiter can mean “Please bring me…”. ² A boss saying “I would like…” should probably be understood as giving a polite command rather than merely expressing a wish. ²

Auxiliaries and their uses

The intended meaning should guide the choice of auxiliary and any accompanying adverb, because each auxiliary can serve several purposes. ⁵ Modal verbs include can, could, may, might, must, shall, should, will and would. ¹ Semimodals such as dare, need, ought to and used to, and expressions such as have (got) to, be going to and be able to, can also carry modal meanings. ¹

The following auxiliary uses distinguish ability, possibility, permission, obligation and intention, alongside the auxiliary uses of do. ^{5, 4, 6}

Auxiliary	Uses and examples
Can	Ability, possibility, probability or permission.
Could	Past uses of can; past conditional uses such as “he said that he would go if he could”; a less certain possibility, as in “That could be true”; and polite requests, as in “I would be grateful if you could turn down the volume on your stereo”.
Do	Emphasis, questions and negation; it can also function as an ordinary verb.
May	Possibility, probability or permission; volition in prayers, curses and blessings (“may peace be upon them!”); purpose or expectation (“I sow so that I may reap”); contingency (“We’ll be prepared, come what may”); concession (“He may be small but he is wiry”); choice (“You may cook it on the stovetop or in the microwave”); and permission without a requirement in legal wording.
Might	Historically the past tense of may; a more tentative possibility (“It might be possible to get there on time”); a contrary-to-fact condition (“if you had done your homework, you might be able to pass the test”); a polite alternative to may (“might I have this next dance?”) or to ought or should (“you might at least pay for the damages”).
Must	Necessity or requirement, a command, what someone should do, or strong likelihood (“That must be Janet calling”).
Shall	Simple futurity, commands or exhortations (“thou shalt not kill”), mandatory legal wording (“Congress shall make no law…”), future inevitability or likelihood (“We shall see”), and determination (“They shall not pass!”).
Should	Historically the past tense of shall, still sometimes used for futurity viewed from the past; condition (“if he should leave his father, his father would die”); obligation, propriety or expediency (“you should brush your teeth after every meal”); probability or expectation; and softened statements or requests.
Will	Futurity; habitual actions or natural tendencies; desire, choice, willingness or consent; refusal in negative constructions (“I won’t!”); capacity or sufficiency (“the tank will hold 40 liters”); probability or inevitability; determination, insistence, persistence or wilfulness; and commands, exhortations or injunctions.
Would	Historically the past tense of will; what someone said or thought was going to happen or be done; imagined situations or things that did not happen; would have can refer to something that did not happen or was not done.

^{4, 6}

The auxiliary do

Do can emphasise an indicative assertion, form a question, negate an assertion or function as an ordinary verb. ^{3, 6} The contrasting examples show these uses. ³

• Emphasis: “I do believe in spooks”, as spoken by the Cowardly Lion in The Wizard of Oz.
• Question: “Do you love me?”
• Negation: “She does not smoke cigarettes.”
• Ordinary verb: “After supper, we did the dishes.”

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