The interpretation of the functional category of Subject in English has always been rather problematic. As we noted above, the definition of Subject inherited from classical times was a morphological one: it was that nominal element – ‘noun or pronoun’ – that is in the nominative case, and that displays person and number concord with the (finite) verb. But few traces remain, either of case in the noun or of person and number in the verb. What made the situation more problematic was that, in the structuralist tradition, the Subject was said to be a purely grammatical element, operating at the syntactic level but without semantic significance. That something should be a grammatical function whose only function is to be a grammatical function is already somewhat anomalous; it becomes even more anomalous if it has no clear syntactic definition.
In the Cardiff Grammar (e.g. Fawcett 2010), Subject is construed as a syntactic category.