Using Kibort's revised Lexical Mapping Theory (Kibort, 2007, 2008, 2013, 2014), this paper gives an analysis of the argument structure of verbal predicates that alternately take (i) a clausal subject and (ii) a subject it in conjunction with propositional subclause. Based on data from syntactically annotated corpora of historical English, two separate argument structures are posited for the relevant predicates. A distinction is made between, on the one hand, a thematic subject it occurring together with an adjunct subclause and, on the other hand, a non-thematic subject it occurring together with a complement subclause. This distinction provides an explanation for a number of facts in present-day and historical English concerning extraction and co-occurrence patterns with respect to the predicates showing the alternation under discussion.