Halliday & Matthiessen (2014: 140):
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 ‘fused’ 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 give; give(s) not do(es) give.