Halliday & Matthiessen (2014: 143n):
Note that the system of MOOD is a system of the clause, not of the verbal group or of the verb. Many languages also have an interpersonal system of the verb(al group) that has been referred to as ‘mood’: it involves interpersonal contrasts such as indicative/subjunctive, indicative/subjunctive/optative. To distinguish these verbal contrasts from the clausal system of MOOD, we can refer to them as contrasts in mode. The subjunctive mode tends to be restricted to the environment of bound clauses — in particular, reported clauses and conditional clauses having the sense of irrealis. In Modern English, the subjunctive mode of the verb is marginal, although there is some dialectal variation.
