The required ordering of verb forms when all five verbs are present is:
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 20.1 by type. The third column of Table 20.1 lists the verb forms that are required to follow each type of auxiliary verb.
modal base perfective progressive passive
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