In the previous section, I briefly introduced you to the modal auxiliaries when I argued that will does not constitute a separate tense marker. To understand the function of modal auxiliaries, you need to know two related terms: modality and mood.

Modality refers to a set of related concepts primarily involving the attitude of the speaker of a sentence towards the reality of a particular assertion. What exactly that means is complicated and best illustrated with an example:

(12a) Tad programs computers for a living.
(12b) Tad must program computers for a living.

In sentence (12a), the speaker asserts the truth of a proposition. In (12b), by contrast, the speaker qualifies the proposition. The situation is presented not as one the speaker knows directly but as one the speaker has inferred. In other words, in (12b), must indicates something about the speaker's mental state. These sentences, therefore, contrast in their modality.

Mood refers to a grammatical system that is primarily used to convey modality. The difference between mood and modality is parallel to the difference between tense and time. Like time, modality is a semantic concept; like tense, mood is a grammatical realization of a concept. For the most part, English expresses mood analytically, through a system of modal auxiliaries.[1] As with tense, mood does not always correspond in a simple fashion with modality. One modal verb can express several different modalities, depending on the context. And just as time can be expressed in different parts of a sentence, for example by prepositional phrases, modality can be indicated with things other than auxiliary verbs:

(13) I heard his supposed apology.

In sentence (13) the adjective supposed expresses the speaker's conclusion that the apology is not a valid one, for example because it lacks sincerity. Words such as supposed, then, express modality, but not mood.

Sentence (12a) represents the default situation, one without a modal verb, in which the speaker simply indicates that something is true. This unmarked situation is called the indicative mood, although since this is the ordinary case, we usually don't mention it unless we're contrasting it with another mood.

In some grammar books, the presence of a modal auxiliary is said to mark the conditional mood. This label reflects the fact that modal auxiliaries commonly appear in sentences that express a condition:

(14) If you build it, they will come.

However, the label conditional is not ideal. There are many other situations in which modal auxiliaries appear other than the conditional structure. Further, many conditional sentences do not use modal auxiliaries:

(15) If he got a ticket to the concert, he was lucky.

Because modal auxiliaries express a variety of different modalities, we will not try to lump them all into a single mood. Instead, we will simply call such verb phrases modal, and if we need to distinguish among them, we will do so by their meaning.


[1] Exceptions to the analytical nature of English mood are the constructions traditionally called the "subjunctive", which are marked on the verb itself. They play a fairly small role in the grammar of English, but are more prominent in languages like Spanish, French, or Latin.

Characteristics of Modal Verbs

There are a small number of modal verbs, and they display distinct features that set them apart from other auxiliary verbs.

The Principal Modal Auxiliaries:

Present Tense Past Tense
can could
may might
must --
ought --
shall should
will would

This set of verbs differs from other auxiliaries in the following ways:

  • They do not agree in the third-person singular, as do other auxiliaries and lexical verbs.

    (16) *She cans play the piano beautifully.

  • They are followed by a bare infinitive form of another verb. Most other verbs use the infinitive with to. Ought is an exception to this rule. It does require a to-infinitive but otherwise behaves like other modal verbs.

    (17a) *They must to work on the project.
    (17b) They want to work on the project.
    (17c) They ought to work on the project.

  • They have no non-finite forms (present participle, past participle or infinitive). As a consequence, they cannot appear in places in the verb phrase where one of these forms would be required:

    (18) *Robertson was shoulding here tonight.
    (19) *The Senate has mayed ignore its own rules.
    (20) *I would like to will take you out to dinner.

A different way putting this last point would be to say that all the modal verbs have an inherent tense, as indicated in the table above. That table is organized in two columns to show you the relationship between present and past tense forms. In other words, would is the past-tense of will, could the past tense of can, etc.

Because modal verbs are specialized function words, the formal realization of tense may not always correspond with time reference. We frequently use all of these verbs to discuss future or potential events, and so these verbs may not intuitively feel like normal present or past tense verbs. But there are important ways in which the tense of these modals remains relevant.

Sentences with multiple verb phrases often establish a consistent tense, either present or past. As we saw in the previous section, the pair will/would participates regularly in this sequence of tenses. So does can/could:

Past-tense Sequence:
(21a) Scientists said that the volcano, which had been dormant for many years, could erupt at any time.
(21b) *Scientists said that the volcano, which had been dormant for many years, can erupt at any time.

Present-tense Sequence:
(21c) Scientist say that the volcano, which has been dormant for many years, can erupt at any time.
(21d) *Scientist say that the volcano, which has been dormant for many years, could erupt at any time.

The situation is more complicated with the pairs shall/should and may/might. In earlier stages of the language, these verbs were once used systematically just as the other two pairs still are. In contemporary English, however, other factors to make the relationship more complex.

Shall, for example, is rare apart from formal contexts, and should has developed uses that are unrelated to its past-tense status. But shall is incompatible with past-tense sequences, and should substitutes for shall in such contexts.

(22a) We shall read Austen during the course.
(22b) We know that we shall read Austen during the course.
(22c) *We knew that we shall read Austen during the course.
(22d) We knew that we should read Austen during the course.
(22e) We know that we should read Austen during the course.

Sentence (22a) is certainly formal, but it is a possible sentence, and when we add another present-tense verb, as in (22b), shall remains a possibility. But when we add a past-tense verb, as in (22c) and (22d), only should is grammatical.

Sentences like (22e) complicate the analysis. In this case, we have should, which I have been arguing is a past-tense verb, in a sequence with present-tense know. In this case, however, should takes on a different meaning. In (22a), shall is more or less equivalent to will. In (22e), it is essentially equivalent to ought to. Notice that (22d) can support either meaning. In the first sense, it indicates future in the past, equivalent to would (i.e., will + the past tense); in the second, it indicates what the speaker believes the right course of action is. This other use for should has no present-tense equivalent, and can be used in either context. There is no particular reason to think that should somehow shifts its tense when it has the meaning ought to and maintains its traditional tense in other cases. Rather, this is the a case of an idiomatic usage that overrides the ordinary patterns of tense usage. Such irregularities are, from the perspective of someone trying to learn the language, unfortunate, but they are a reality in all languages.

The usage of may and might is currently in flux. The traditional distinction, using might as a past-tense equivalent to may persists in some varieties of English. In others, however, speakers have reanalyzed the two verbs as separate, unrelated forms. To tell which variety you speak, examine the following sentences. How would you judge the grammaticality of (23a)?

(23a) ?I knew he may let us down.
(23b) I knew he might let us down.

If (23a) sounds strange to you, your variety of English still preserves, at least in some contexts, might as a past-tense equivalent of may. If they both sound acceptable, then your variety of English no longer treats these verbs as a related pair. The fact that sentences like (23a) were historically, and for many speakers still are, ungrammatical is sufficient evidence for us to continue to classify may and might as related pairs. Lending further support to this choice, there are some uses of might that reflect "modal remoteness", a concept that will be explained in the next section.

The remaining two verbs in our list, must and ought have no past-tense equivalents, so they do not participate at all in the alternation of tenses described above for the other modal verbs. One interesting point to note about both verbs is that historically, they derive from the past tenses of other verbs. Over time, however, speakers of English have reanalyzed them so that now they behave purely as present-tense verbs.

(24a) *She must finish the project two days ago.
(25a) *You ought to visit your mother last week.

Neither verb is compatible with a past-time reference by itself. If we want to use must or ought in a past-time context, we have to use an alternate method of indicating past time:

(24b) She must have finished the project two days ago.
(25b) You ought to have visited your mother last week.

This method, known as the perfect, will be explained later in the chapter.