Pro-Drop, also known as the ‘Null Subject’ phenomenon, is a well-known property of many (perhaps most) natural languages. Admittedly, there is some vagueness in the way it is used in the literature, oftentimes with any absence of a full or weak subject pronoun in a sentence being referred to as a ‘null subject’ and with other properties, e.g. the appearance or absence of nonreferential, expletive, pronouns as subjects in such constructions as weather verbs, extrapositions, and the like, taken to be diagnostic for Pro-Drop, more so even than the possibility of null subjects in other constructions; still, the Null Subject Parameter can be defined as in (1), based on Jaeggli & Safir (1989), who drew on the ‘Subject Requirement Constraint’ of Perlmutter (1971).
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