To summarise, there are five forms in which an English verb can appear: the base form (uninflected), the past tense form, the third person singular present form, the perfective (and passive) form and the progressive fo
What determines the use of the auxiliary here? Obviously the verb is unable to support the inflection in this case, but this does not seem to be because it already supports another morpheme. In fact the verb is in its base form and there is no reason to think that there is any other verbal morpheme present. (21a) is simply the negative version of he arrived. Apparently it is the negative that blocks the verb from moving to support the inflection. To gain some understanding of what is going on here we need to briefly examine another kind of head movement which we will more thoroughly discuss in the next chapter. In the formation of certain questions an auxiliary verb is moved to the other side of the subject:
The obvious question is what are these auxiliaries supporting? Note that any element that appears after a free inflectional element is always in its base form. Thus, either the auxiliaries are supporting nothing, which throws doubt on their treatment as inserted empty elements, or they are supporting a null morpheme. The latter assumption allows us to maintain our approach but it raises the subsequent question of what this morpheme is.