The grammatical system has … a functional input and a structural output; it provides the mechanism for different functions to be combined in one utte… - Michael Halliday

What makes learning possible is that the coding imposed by the mother tongue corresponds to a possible mode of perception and interpretation of the environment. A green car can be analysed experientially as carness qualified by greenness, if that is the way the system works.

The interpersonal function [of language] is the function “to establish, maintain, and specify relations between members of societies”

The theme is what is being talked about, the point of departure for the clause as message, and the speaker has within certain limits the option of selecting any element in the clause as thematic.