Modal auxiliary verbs

Modal auxiliary verbs express meanings such as obligation, possibility, necessity, etc., which reflect somebody's (often the writer's) attitudes towards the state, event, etc. expressed by the verb phrase. Often, it is difficult to pinpoint the exact type of meaning conveyed by a modal auxiliary, as different types of meaning may be present simultaneously. For example, among other things, the modal can may be used to express ability and possibility, as in the following examples:

(1) Bill can swim. (i.e. 'Bill has the ability to swim') (2) The door can only be opened with a special key. (i.e. 'it is only possible to open the door with a special key')

Fairly often, however, it is difficult to claim that only one of these meaning is expressed:

(3) I can drive you to the airport tomorrow. ('I have the ability to drive you, e.g.  because I have a driver's licence' or 'It is possible for me to drive you, e.g. because I am free at the appropriate time')

Grammatical properties of modal auxiliaries

In addition to the the properties shared by all auxiliaries, modals are characterized by the following:

• Modals have no non-finite forms (i.e. they have no infinitival or participial forms).
• Modals have no inflected forms in the present tense (in particular, they have no 3rd person -s ending).

The fact that modals have no non-finite form limits the possibility to combine them with other auxiliaries. For example, they do not occur as the second verb in the perfect, in contrast with other auxiliaries, like be:

(3) *Bill has never could drive a car. (cf. Bill has never been kissed.)

In cases where a modal auxiliary is impossible due to the lack of appropriate forms, English uses alternative expressions, as in the following examples:

Be able to instead of can:
(4) Bill has never been able to drive a car. (5) We hope to be able to solve this problem in the future. (6) Being able to solve simultaneous equations is useful for a number of reasons. Be allowed to instead of can/may (expressing permission)
(7) Mary has never been allowed to listen to jazz music. (8) To be allowed to enter the competition every contestant must have a clean medical record. (9) Everyone was required to pay a special tax before being allowed to vote.

The fact that modals have no inflected forms in the present tense make them useful for non-native speakers who often experience difficulties with Subject-Verb agreement. Thus, as the following illustrates, the present tense form can is used for all subjects, whereas other auxiliaries, like be, and main verbs, likehate, vary in form.

Modal auxiliary	Primary auxiliary	Main verb
I can call Bill tomorrow.	I am calling Bill now.	I call Bill every day.
James can call Bill tomorrow.	James is calling Bill now.	James calls Bill every day.
We can call Bill tomorrow.	We are calling Bill now.	We call Bill every day.