1.3.6 Functionally underspecified categories
This would suggest that they are not categorised in the same way as inflections. They appear to be verbal elements as they inflect for almost the same set of things that verbs do (perfective have inflects for tense (has/had), progressive be inflects for tense (is/was) and perfect aspect (been) and passive be inflects for tense (is/was), perfect aspect (been) and progressive aspect (being)). However, they are clearly not thematic elements in that they play no role in the thematic interpretation of the sentence. Aspectual auxiliaries therefore share properties with verbs and inflections, but they cannot be categorised as either. We can capture this situation if, like verbs and inflections, we categorise aspectuals as [–N, +V] elements, but simply leave the [±F] feature undefined. In common with inflections, aspectual auxiliaries also take only verbal complements and they never precede any other category. However, they may precede either verbs or other aspectual auxiliaries: