The question amounts to that of how we decide what to say - what kind of speech act we wish to make and what propositional content we need to include. Bock [Bock, J. K., 1982, “Toward a cognitive psychology of syntax: Information processing contributions to sentence formulation, “Psychological Review” 89, pp. 1-47] termed this the "interfacing representation" - that which interfaces between language and thought.
- Dell (2004), a pag.131