Auxiliaries

Although there has been some debate about the lexical category of auxiliaries, the English XTAG grammar follows [#!mccawley88!#], [#!haegeman91!#], and others in classifying auxiliaries as verbs. The category of verbs can therefore be divided into two sets, main or lexical verbs, and auxiliary verbs, which can co-occur in a verbal sequence. Only the highest verb in a verbal sequence is marked for tense and agreement regardless of whether it is a main or auxiliary verb. Some auxiliaries (be, do, and have) share with main verbs the property of having overt morphological marking for tense and agreement, while the modal auxiliaries do not. However, all auxiliary verbs differ from main verbs in several crucial ways.

• Multiple auxiliaries can occur in a single sentence, while a matrix sentence may have at most one main verb.
• Auxiliary verbs cannot occur as the sole verb in the sentence, but must be followed by a main verb.
• All auxiliaries precede the main verb in verbal sequences.
• Auxiliaries do not subcategorize for any arguments.
• Auxiliaries impose requirements on the morphological form of the verbs that immediately follow them.
• Only auxiliary verbs invert in questions (with the sole exception in American English of main verb be^21.2).
• An auxiliary verb must precede sentential negation (e.g. John not goes).
• Auxiliaries can form contractions with subjects and negation (e.g. he'll, won't).

The restrictions that an auxiliary verb imposes on the succeeding verb limits the sequence of verbs that can occur. In English, sequences of up to five verbs are allowed, as in sentence ((385)).

(384)0(384

(385)

The music should have been being played [for the president] . 

The required ordering of verb forms when all five verbs are present is:

modal base perfective progressive passive

The rightmost verb is the main verb of the sentence. While a main verb subcategorizes for the arguments that appear in the sentence, the auxiliary verbs select the particular morphological forms of the verb to follow each of them. The auxiliaries included in the English XTAG grammar are listed in Table 21.1 by type. The third column of Table 21.1 lists the verb forms that are required to follow each type of auxiliary verb.

TYPE	LEX ITEMS	SELECTS FOR
modals	can, could, may, might, will,	base form21.1
	would, ought, shall, should	(e.g. will go, might come)
	need
perfective	have	past participle
		(e.g. has gone)
progressive	be	gerund
		(e.g. is going, was coming)
passive	be	past participle
		(e.g. was helped by Jane)
do support	do	base form
		(e.g. did go, does come)
infinitive to	to	base form
		(e.g. to go, to come)

{Auxiliary Verb Properties