VERBAL MORPHOLOGY the syntax of lexical verbs vs. auxiliaries.
Before X’ theory, sentences used to be identified with the notation S. Linguists had to figure out a way of replacing S by a phrasal category. In other words, they needed to identify some elements as the head of the overall sentence.
Since phrases were (or are) projections of heads, consisting specifically of three levels, it was necessary to find a category that could act as the head of the new phrase.
The category in question would have to be associated in some way to verbs, since verbs are actually elements that can make up a sentence on their own that is without the need of any other constituent. As is well known, imperatives give evidence of this: Go!
INFLECTION, that is, verbal inflection, actually played a major role in a basic distinction affecting sentences in very many languages. We need to offer a characterisation of I and also specify the distinction that exists between so-called ‘sentences’ on the on hand and ‘clauses’ in the other.
INFLECTION is any type of grammatical affix that is attached to a root. Thus, in a language like English, the suffix or ending –er that is attached to adjectives to make the comparative form is identified as I (inflection), specifically adjectival I. Likewise, the plural ending –s that appears on nouns is an instance of nominal I.
Regarding verbs it is necessary to distinguish between finite or tensed I on the one hand, and non-finite or untensed I on the other hand. Finite I expresses agreement and tense. Agreement is identified as person and number and tense is [+ Present] or [-Present] for a language like English. We cannot confuse tense (which is a grammatical or functional concept) with time (which is a sentence concept).
take-s 3rd pers.sing., +pres. walk-0 1st or 2nd pers. sing., 1st, 2nd, 3rd pers. plur., +pres. knock-ed 1st, 2nd or 3rd pers. sing. or plur., -pres. taught 1st, 2nd, or 3rd pers. sing. or plur.,