Halliday & Matthiessen (2014: 411):
There is no system of mood in the verbal group. If a clause is ‘free: indicative’ or ‘bound: finite’, the verbal group is ‘finite’. If a clause is either ‘free: imperative’ or ‘bound: non-finite’ the verbal group is ‘non-finite’. The verbal group of an imperative clause is ‘perfective’ in aspect and ‘no secondary’ or ‘present’ in (secondary) tense; negative variants have the special realisation don’t. (In various languages, clausal contrasts in MOOD correlate with verbal contrasts in MODE.)