This is the only sub-class of `verb' that is recognised (as a word-class) in WG, apart from its own sub-class `modal verb' and (possibly) the remaining verbs, which we can call `full verbs'. Its members are:
- BE (all uses, even when not followed by a verb)
- HAVEperfect and, for some speakers, HAVEpossess
- all modal verbs (CAN, MAY, WILL, SHALL, MUST, OUGHT, USED and a few others)
It is a beautiful word-class because it is so clearly defined by so many characteristics (most of which, incidentally, have developed since the 14th century - our auxiliary verbs are much more clearly defined than Shakespeare's! See Hudson 1990). Here are its distinctive characteristics.
- Only (tensed) auxiliary verbs undergo subject-auxiliary inversion:
(1) Are/*get they tired?
- Only (tensed) auxiliary verbs allow a dependent NOT to follow them:
(2) They are/*get not tired.
- Only (tensed) auxiliary verbs accept n't.
(3) They aren't/*getn't tired.
- Only auxiliary verbs have reduced forms (e.g. is ~ 's), though not all auxiliary verbs do have shortened forms, and shortened forms tend to be tensed.
(4) They're/*Theyg't tired.
(5) They may've been tired.
- Only auxiliary verbs allow `partial ellipsis' - omission of the complement verb, but retention of its object. Many full verbs allow full ellipsis, which is also possible with auxiliary verbs.
(6) I don't know whether he likes grammar, but he may/tries. (full ellipsis)
(7) I don't know whether he likes grammar, but he may/*tries phonology.
(partial ellipsis, meaning `he may like phonology' or `he tries to like phonology')
Both subject-auxiliary inversion and the addition of -n't are handled as inflections of the auxiliary verb.
The name `auxiliary verb' is highly inappropriate to this word-class, because only one of these characteristics (the last one) has anything to do with the presence or absence of another verb which this one `helps' (Latin AUXILIUM, `help'). Indeed, some auxiliary verbs (as defined by these characteristics) may be used without any other verb, overt or ellipsed.
(8) Is she here?
(9) Has she time?
In both these examples the single verb (is, has) has an inverted subject, so it must be an auxiliary verb. My own preferred term is `polarity verb', but it seems perverse not to use the established term, especially since these verbs are most typically used to support other verbs. Please bear this in mind in deciding how to classify a word - the only relevant question is whether it allows subject-auxiliary inversion, accepts -n't, etc.
Another characteristic shared by most auxiliary verbs is that they have very little `referential' meaning, i.e. meaning you could define in terms of characteristics of situations in the world, which helps you to know what the speaker is talking about; e.g. in (10) the main distinctive load is carried by wash, not by may.
(10) Pat may wash it.
In some cases the semantic relation between the auxiliary and the next verb is co-reference; e.g. in will come or did come the two verbs both refer to the same event (with the auxiliary defining its time, like a tense inflection). In spite of this, the auxiliary verb is syntactically the parent of the next verb, which is its complement. (This is perfectly normal for a parent-dependent pattern, as explained in dependency.) I.e. the syntactic structure of (10) is just the same as that of (11) and (12), where the first verb is not an auxiliary verb.
(11) Pat helps wash it.
(12) Pat keeps washing it.
These examples illustrate the irrelevance of the notion `auxiliary' which I grumbled about in the previous paragraph: in each case the first verb `helps' the second one, by supporting it syntactically, so there is nothing unusual about so-called auxiliary verbs from this point of view. All such verbs take a non-finite verb as their `sharer', which shares their subject. The figure illustrates the pattern.
Auxiliary verbs include the class of modal verbs, as shown in the next figure.