Sentence : a Larger Phrase

S = NP + VP
There is no construction like usual phrase with head, specifier and complement.
I take the umbrella
NP VP
But?
I will take the umbrella
S = NP + Inflection (Infl) + VP
Inflection : tense marker or auxiliary verb

A scientist discovered the answer

NP: A scientist, Inf: past, and VP: discover the answer


A sentence has an abstract category dubbed “Infl,” which indicates the tense. Because Infl, like all heads, is obligatory, this automatically accounts for the fact that all sentences of English have tense.
Sometimes, an actual word can appear in the Infl position. Auxiliary verbs may be in the head position of sentences and are thus treated as an instance of the Infl category.
Sentences, thus, have Infl as their head and that Infl may be realized as either a tense label or an auxiliary.

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