Functional Discourse-Grammar or FDG is the latest development in the functional grammar that was initiated by the Dutch linguist Simon Dik (1940-1995). In this paper, the FDG architecture proper is described, including the role of the extra-grammatical conceptual and contextual components. A simple interrogative clause in Esperanto is used to illustrate how a linguistic expression is built up from the formulation of its (pragmatic) intention to its articulation. Attention is paid to linguistic transparencies and opacities, defined here as the absence or presence of discontinuities between the descriptive levels in the grammar. Opacities are held accountable, among other factors, for making languages more or less easy to learn. The grammar of every human language is a complex system. This is clearly demonstrable precisely in Esperanto, in which the relatively few difficulties, identified by the opacities in the system, form such a sharp contrast to the general background of freedom, regularity and lack of exceptions.