Halliday & Matthiessen (2014: 139-40):
When we come to look closely at statements and questions, and at the various responses to which these naturally give rise, we find that in English they are typically expressed by means of a particular kind of grammatical variation: variation which extends over just one part of the clause, leaving the remainder unaffected. … one particular component of the clause is being, as it were, tossed back and forth in a series of rhetorical exchanges; this component carries the argument forward. Meanwhile the remainder … [can be] simply left out, being taken for granted as long as the discourse continues to require it. …
What is the component that is being bandied about in this way? It is called the Mood element, and it consists of two parts: (1) the Subject, which is a nominal group, and (2) the Finite operator, which is part of a verbal group.