Contracted forms

Unlike be and have (but not do), modals in the affirmative do not have contracted forms, except for will and would [I'll, I'd > 9.35, 14.17n3]. In speech, can, could and shall are 'contracted' by means of unemphatic pronunciation:

/, (etc.) can /kən/, /, (etc.) could /kəd/, I/We shall /ʃəl/

One modal at a time

Only one modal can be used in a single verb phrase:

We may callthe doctor but not may and must together.

We must callthe doctor

If we wish to combine the two ideas in the above sentences, we have to find a suitable paraphrase:

It may be necessary(for us) to calla doctor By comparison, we can use e.g. be and have together:

It has beennecessary to call a doctor

Form of modal auxiliaries compared with future tenses

Each of the modals fits into the four patterns for future tense forms:

/ will see / will be seeing / will have seen / will have been seeing modal + (bare infinitive): modal + be + present participle: modal + have + past participle:

simple future [> 9.35]
future progressive [> 9.40]
future perfect simple [> 9.42]
future perfect progressive [> 9.42]
active passive

/ may see I may be seen

/ may be seeing - [but > 12.3n.6]

/ may have seen I may have been seen

modal + have been + present participle:/ may have been seeing