Primary function of modal verbs
In their primary function, modal verbs closely reflect the meanings often given first in most dictionaries, so that:
- can/could relate mainly to ability: / can lift 25 kg/I can type
- may/might relate mainly to permission:You may leaveearly
- will/would relate mainly to prediction[> 9.35]: it will rainsoon
- shall after I/We [> 9.36n1] relates mainly to prediction:Can we find our way home? - I'm sure we shall
- should/ought to relate mainly to escapable obligationor duty:You should do(or ought to do) as you're told
- must relates mainly to inescapable obligation:You must bequiet
- needn't relates to absenceof obligation:You needn't wait
11 Modal auxiliaries and related verbs
Secondary function of modal verbs
In their secondary function, nine of the modal auxiliaries (not shall) can be used to express the degree of certainty/uncertainty a speaker feels about a possibility. They can be arranged on a scale from the greatest uncertainty {might) to the greatest certainty {must). The order of modals between might and must is not fixed absolutely. It varies according to situation. For example, one arrangement might be:
might very uncertain
may
could
can'
You should be right
ought to have been right
would
will
must almost certain
| You |
| are |
| right |
Certain
Can requires qualification to be used in this way [> 11.29ns2.4]: He can hardly be right Do you think he can beright7 I don't think he can beright
Primary and secondary functions of 'must' compared
This example of must shows that it is 'defective' [> 11.6.1]:
1 In its primary function it requires another full verb {have to) to make up its 'missing parts'. (In the same way can, for example, in its primary function requires the full verb be able to to make up its missing parts.)
2 In its secondary function must (like the other modals listed in 11.1) has only two basic forms: a form which relates to the present and a form which relates to the perfect or past [> 11.8.4],
| They must beright |
| They must have beenright |