The subject is the independent member of a two-member predication, containing the person component of predicativity.
The subject is generally defined as a word or a group of words denoting the thing we speak about. The subject of a simple sentence can be a word, a syntactical word-morpheme or a complex.
As a word it can belong to different parts of speech, but it is mostly a noun or a pronoun. A word used as a subject combines the lexical meaning with the structural meaning of “person”. So it is at the same time the structural and the notional subject.
The syntactical word-morphemes there and it may also function as secondary subjects (It being cold, we put on our coats. I knew of there being no one to help them). The analysis of sentences like He was seen to enter the house, is a point at issue. Traditionally the infinitive is said to form part of the complex subject (He…to enter). Ilyish maintains that though satisfactory from the logical point of view, this interpretation seems to be artificial grammatically, this splitting of the subject being alien to English. He suggests that only HE should be treated as a subject, whereas was sees to enter represents a peculiar type of compound predicate. Some grammarians (Smirnitsky, Ganshina) speak of definite-personal, indefinite-personal, impersonal sentences, but it is a semantical classification of subjects, not sentences.