Structural Realisations Of Indicative Mood

Halliday & Matthiessen (2014: 143):

(1) The presence of the Mood element, consisting of Subject plus Finite, realises the feature ‘indicative’.

 (2) Within the indicative, what is significant is the order of Subject and Finite:

(a) the order Subject before Finite realises ‘declarative’;
(b) the order Finite before Subject realises ‘yes/no interrogative’;
(c) in a ‘WH- interrogative’ the order is:

(i) Subject before Finite if the WH- element is the Subject;
(ii) Finite before Subject otherwise.