the MOOD [...] consists of two parts: (1) the Subject, which is a nominal group, and (2) the Finite element, which is part of a verbal group.
- Halliday (1985), a pag.72

Subject and Finite are closely linked together, and combine to form one constituent which we call the Mood [...] The Mood is the element that realizes the selection of Mood in the clause. It has sometimes been called the ʻModalʼ element; but the difficulty with this is that the term 'modal' is ambiguous, since it corresponds both to 'mood' and to 'modality'. The remainder of the clause we shall call the Residue.
- Halliday (1985), a pag.73-74

[...] The Mood element has a clearly defined semantic function: it carries the burden of the clause as an interactive event. So it remains costant, as the nub of the proposition, unless some positive step is taken to change it, as in The duke has given your aunt a new teapot, hasn’t he? (i) No, he hasn’t. But (ii) - (a) the duchess has. - (b) he’s going to. Here the proposition is first disposed of, by being rejected, in (i); this then allows for a new proposition, with change of Subject, as in (a), or change of Finite, as in (b). Each of these two constituents, the Subject and the Finite, plays its own specific and meaningful role in the propositional structure.
- Halliday (1985), a pag.77