Modal Auxiliary Verbs  

A modal auxiliary verb, often simply called a modal verb or even just a modal, is used to change the meaning of other verbs (commonly known as main verbs) by expressing modality—that is, asserting (or denying) possibility, likelihood, ability, permission, obligation, or future intention.

Modal verbs are defined by their inability to conjugate for tense and the third person singular (i.e., they do not take an “-s” at the end when he, she, or it is the subject), and they cannot form infinitives, past participles, or present participles. All modal auxiliary verbs are followed by a main verb in its base form (the infinitive without to); they can never be followed by other modal verbs, lone auxiliary verbs, or nouns.

As with the primary auxiliary verbs, modal verbs can be used with not to create negative sentences, and they can all invert with the subject to create interrogative sentences.

There are nine “true” modal auxiliary verbs: will, shall, would, should, can, could, may, might, and must. The verbs dare, need, used to, and ought to can also be used in the same way as modal verbs, but they do not share all the same characteristics; for this reason, they are referred to as semi-modal auxiliary verbs, which are discussed in a separate section.

We’ll give a brief overview of each modal verb below, but you can continue on to their individual sections to learn more about when and how they are used.

As a modal auxiliary verb, will is particularly versatile, having several different functions and meanings. It is used to form future tenses, to express willingness or ability, to make requests or offers, to complete conditional sentences, to express likelihood in the immediate present, or to issue commands.

The modal auxiliary verb shall is used in many of the same ways as will: to form future tenses, to make requests or offers, to complete conditional sentences, or to issue maxims or commands. Although will is generally preferred in modern English, using shall adds an additional degree of politeness or formality to the sentence that will sometimes lacks.

Generally, shall is only used when I or we is the subject, though this is not a strict rule (and does not apply at all when issuing commands, as we’ll see).

The modal auxiliary verb would has a variety of functions and uses. It is used in place of will for things that happened or began in the past, and, like shall, it is sometimes used in place of will to create more formal or polite sentences. It is also used to express requests and preferences, to describe hypothetical situations, and to politely offer or ask for advice or an opinion.

The modal verb should is used to politely express obligations or duties; to ask for or issue advice, suggestions, and recommendations; to describe an expectation; to create conditional sentences; and to express surprise. There are also a number of uses that occur in British English but are not common in American English.

As a modal auxiliary verb, can is most often used to express a person or thing’s ability to do something. It is also used to express or ask for permission to do something, to describe the possibility that something can happen, and to issue requests and offers.

The modal verb could is most often used as a past-tense version of can, indicating what someone or something was able to do in the past; it can also be used instead of can as a more polite way of making a request or asking for permission. Could is also used to express a slight or uncertain possibility, as well as for making a suggestion or offer.

The modal verb may is used to request, grant, or describe permission; to politely offer to do something for someone; to express the possibility of something happening or occurring; or to express a wish or desire that something will be the case in the future. We can also use may as a rhetorical device to express or introduce an opinion or sentiment about something.

The modal verb might is most often used to express an unlikely or uncertain possibility. Might also acts as a very formal and polite way to ask for permission, and it is used as the past-tense form of may when asking permission in reported speech. It can also be used to suggest an action, or to introduce two differing possibilities.

The modal verb must is most often used to express necessity—i.e., that something has to happen or be the case. We also use this sense of the word to indicate a strong intention to do something in the future, to emphasize something positive that you believe someone should do, and to rhetorically introduce or emphasize an opinion or sentiment. In addition to indicating necessity, must can be used to indicate that something is certain or very likely to happen or be true.

Modal auxiliary verbs are used to uniquely shift the meaning of the main verb they modify, expressing things such as possibility, likelihood, ability, permission, obligation, or intention. As we will see, how and when we use modal verbs greatly affects the meaning of our writing and speech.

Modal verbs attach differing shades of meaning to the main verbs they modify. It is often the case that this difference in meaning is or seems to be very slight. To get a better sense of these differences in meaning, let’s look at two sets of examples that use each of the modal verbs we discussed above in the same sentence, accompanied by a brief explanation of the unique meaning each one creates.

As we can see from the above sets of examples, the different modal verbs often have very similar meanings, and it’s sometimes unclear when it is appropriate to use one instead of another. To explore the subtle differences in meaning that occur when we substitute certain modal verbs, go to the section on Substituting Modal Verbs.

A modal verb must always be used with a main verb—they cannot stand completely on their own.

However, it is possible to use a modal verb on its own by omitting the main verb, so long as it is implied by the context in or around the sentence in which the modal is used. This can occur when a sentence is in response to another one, or when the clause with the modal verb occurs later in a sentence in which the main verb was already stated. For example:

Generally speaking, we use adverbs after a modal verb and either before or after the main verb in a clause. Sometimes putting an adverb before a modal is not incorrect, but it will sound better if placed after it. For example:

However, this is not a strict rule, and certain adverbs are able to go before the modal verb without an issue. For example:

When a modal verb is made negative, though, it is sometimes the case that an adverb must go before the modal verb. For example:

Unfortunately, there is no rule that will explain exactly when one can or cannot use an adverb before a modal verb—we just have to learn the correct usage by seeing how they are used in day-to-day speech and writing.

Remember, a modal verb is only used before a main verb, or sometimes before be or have when they are used to create a verb tense. We do not use a modal verb before auxiliary do, or in front of other modal verbs. For example:

When main verbs function on their own, we conjugate them to reflect the third-person singular (usually accomplished by adding “-s” to the end of the verb).

However, we do not conjugate modal verbs in this way, nor do we conjugate a main verb when it is being used with a modal.

For example:

Similarly, we cannot use modal verbs with main verbs that are in a past-tense form; the verb that follows a modal must always be in its base form (the infinitive without the word to). Instead, we either use certain modal verbs that have past-tense meanings of their own, or auxiliary have to create a construction that has a specific past-tense meaning.

For example:

As we saw above, all modal auxiliary verbs must be followed by the base form of the main verb. Just as we cannot use a modal verb with a main verb in its past-tense form, we also cannot use a modal verb with an infinitive: