One of the projects that I have the privilege of taking part in has to do with the development of teaching material for elementary levels of school. I have a small but not unimportant role in advising and commenting on technical issues around language and grammar. I take no credit for the excellent work which was really begun and carried to its present state largely by a single, impressive, individual (whose permission I have not sought to name here).
I'm currently examining the difference between realis and irrealis moods as they relate to practical and creative use of language.
This technical distinction is subtle but I'm thinking that it can justifiably describe language acquisition in a natural, autosegmental way—by "autosegmental" I mean that its sequencing rule (first realis and then irrealis) may be significant to describing language acquisition rules. The realis mood situates and positions the speaker-learner in a context of what they can perceive as the "background" (ie, everything else that "is not").
There is but one notion of a realis mood: the 'to be' phrase.
I am sitting down and typing.
You read my work (ie, you (did) read my work - past tense).
The day (it) was getting longer.
No matter the tense (past, present or infintive), the realis mood merely states/describes a fact (or, a plausibility of it). No matter the language, there is no 'future' realis mood.
A single-worded statement like 'me' obeys a certain set of rules that compel its membership to the category in which 'I am...' statements belong. The realis mood is so amenable to a question like, 'What is this?', that it is the "default" mood in which babies first become aware of the notion of "linguistic competence". This grammatical function's spartan-bare simplicity belies its immense generative capacity: every, single utterance ever spoken by a human being is built up from its foundations.
The irrealis mood, on the other hand, is a real chameleon. It may propose, cajole, beguile, abuse, or do whatever its source and master may have the capacity to muster. Mastery of the irrealis is a mark of greatness that founds all human genius.
If the realis mood be primal and necessary, then the irrealis mood is secondary and contingent. This "natural" stratification and its notion of priority (and only these) make all human language ecologically-rich and adaptive beyond imagining.