[…] 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

The Finite element is one of a small number of verbal operators expressing tense (e.g. 'is', 'has') or modality (e.g. 'can', 'must') [...] Note, however, that in some instances the Finite element and the lexical verb are ʻfusedʼ into a single word, e.g. 'loves'. This happens when the verb is in simple past or simple present (tense), active (voice), positive (polarity) and neutral (contrast): we say 'gave', not 'did gave'; 'give(s)' not 'do(es) give'.
- Halliday (1985), a pag.72

Subject and Finite are closely linked together, and combine to form one constituent which we call the Mood.
- Halliday (1985), a pag.73

Typically, in English, polarity is expressed in the Finite element; each Finite verbal operator has two forms, one positive 'is', 'was', 'has', 'can', etc., the other negative 'isn’t', 'wasn’t', 'hasn’t', 'can’t' (or 'is not', 'cannot' . . .) etc. [...] this is the reason why the Finite element is thematic in a yes/no interrogative clause: such a clause is precisely a request for information regarding polarity. The Finite element is inherently either positive or negative; its polarity does not figure as a separate constituent. It is true that the negative is realized as a distinct morpheme 'n’t' or 'not'; but this is an element in the structure of the verbal group, not in the structure of the clause.
- Halliday (1985), a pag.85-86