Primary (inescapable obligation) secondary (certainty)

infinitive:	to have to leave
-ing form:	having to leave
present.	They must leave
future:	They must leave tomorrow
perfect:	They have had to leave
past:	They had to leave
past perfect:	They had had to leave
future perfect:	They will have had to leave
'conditional':	They would have had to leave

11.5 Some ways in which modals resemble 'be', 'have', 'do'

Structurally, modal auxiliaries resemble the auxiliaries be, have and do in some ways and differ completely from them in others. Some of the most important similarities are noted in this section and some differences are explained in 11.6.

General characteristics of modal verbs

11.5.1 The negative[> 13.1-2]

The negative is formed (as it is for be, have and do) by the addition of not after the modal. In informal spoken English not is often reduced to the unemphatic n't:

be	(is) not	(is)n't	[> 10.6, 10.8]
have	(have) not	(have)nt	[> 10.27-28]
do	(do) not	(do)n't	[> 10.41-42]
can	cannot	can't
could	could not	couldn'/
may	may not	mayn 't
might	might not	mightn't
will	will not	won't	[> 9.35]
would	would not	wouldn't
shall	shall not	shan't	[> 9.36n.3]
should	should not	shouldn't
must	must not	mustn t
ought to	ought not to	oughtn 't to
need	need not	needn 't
dare	dare not	daren't

The full form cannot is written as one word.

Mayn't is rare, but does occur. For used not and usedn't [> 11 59n2],

11.5.2 Questions[> 13.1-3, 13.30, 13.41]

Yes/No questionsare formed as for be, have and do. We begin with the modal, followed by the subject and then the predicate.

May we leave early? In question-word questions,the question-word precedes the modal:

When may we leave? With Yes/No questions, the modal used in the answer is normally the same as the one used in the question [> 11.31, 13.6n.1]:

Can you come and see me tomorrow? - Yes I can'No, I can'tModals also behave like be have and do in tag questions[> 13.17]:

You cando it, can'tyou?

11.5.3 Negative questions[> 13.14]

As with be, have and do, the full form of negative questions with modals requires not after the subject (Can you not help me?). This is formal and rare. Contracted forms are normally used:

Can't you help me? [compare > 13.16] Shouldn't (you) ? is usually preferred to Oughtn't (you) to? perhaps because the latter is more difficult to pronounce. Negative questions with Used? on the above patterns are rare [> 11.59].

11.6 Some ways in which modals differ from 'be', 'have', 'do'

11.6.1 'Defective verbs'

Modals are sometimes called defective verbsbecause they lack forms ordinary full verbs have [> 11.4]. For example: 1 Modals cannot be used as infinitives (compare to be, to have to do). If ever we need an infinitive, we have to use another verb: If you want to apply for this job, you have to be able totype at least 60 words a minute (Not *to" before can or can alone)

11 Modal auxiliaries and related verbs

2 We do not use a to-infinitive after modals (compare be to, have to). Only the bare infinitive [> 16.3] can be used after modals (except ought, which is always followed by to): You must/mustn't phonehim this evening (Not "to phone*)

3 Modals have no -ing form (compare being, having, doing). Instead of -ing, we have to use another verb or verb-phrase: / couldn't go/I wasn't able to go home by bus, so I took a taxi (= Not being able to go...)

4 Modals have no -(e)s in the 3rd person singular (compare is has does): The boss can seeyou now (No -s on the end of can)

5 Each modal has a basic meaning of its own. By comparison, as auxiliaries, be/have/do have only a grammatical function [> 10.1].