As far as I know, the first auxiliary is normally treated as the head of a verb phrase (VP) in the X-Bar theory.

He was writing a letter.

In this sentence, for example, the first auxiliary, was, is normally treated as the head of the VP, was writing a letter, in the X-Bar theory.

In some respect, however, the first auxiliary, was, is merely indicating the tense and aspect of the following verb writing.

Therefore, I wonder if the first auxiliary was can be treated as the specifier of the VP and the following lexical verb writing as the head of the VP in some variant of the X-Bar theory, or if this kind of treatment is a no go in any variant thereof.

1 Answer 1


You are right that the auxiliary is merely a TAM carrier, it's a function word without any meaning of its own. It's however completely logical to take it to be the structural head of the "verb phrase" (the whole IP). People often confuse categorial and functional heads. The auxiliary is the head of the IP because we can "observe" this constituent in the sentence. However the functional head is the V because it's the (only) content word giving the IP its meaning. (The situation would be a little more complicated in the presence of a light verb.)

In sum, it's safe to say that taking the auxiliary to be a specifier would be (as you put it) "a no go in any variant of X' theory", at least as far as English is concerned.

  • Thanks. So the only specifier of a VP in any X' theory has to be an adverb phrase?
    – JK2
    Mar 19, 2018 at 2:20
  • According to Wikipedia, an adverbial is an adjunct, not a specifier; but then it says that the specifier of a verb phrase is a quantifier, such as 'each' or 'all', which is lunacy -- those are appositives of the subject.
    – amI
    Mar 20, 2018 at 23:44
  • @JK2 As mentioned by aml, adverb phrases are adjuncts.
    – Atamiri
    Mar 21, 2018 at 12:20
  • @amI Floating quantifiers can be specifiers, they create nonprojectivity so they can't be attached directly to their NPs in phrase-structure grammars.
    – Atamiri
    Mar 21, 2018 at 12:22
  • In, He definitely was writing a letter, isn't definitely the specifier of the VP? If an adverb cannot be the specifier of a VP, could you tell me what can be along with some example?
    – JK2
    Mar 22, 2018 at 1:43

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