One of the distinctive features of the theory of Functional Discourse Grammar (FDG) is that it distinguishes various hierarchically organized layers of analysis, each of which is provided with a slot for modifiers relevant at the layer in question. Linear placement of these modifiers is determined by the layer to which they belong, with the ordering of elements taking place in a top-down, outward-inward manner. The model, in other words, makes predictions concerning the (relative) clausal position of different types of modifiers. In addition, the model places constraints on the occurrence of different types of modifiers in embedded environments (e.g. within the complement of certain types of verbs). Finally, the model places constraints on which modifiers can be expected to co-occur in coordinated constructions. The aim of the present paper is to test these predictions by examining the function, position and distribution of ten adverbs expressing various kinds of modality. Data from the British National Corpus (BYU-BNC) is used (i) to determine whether the predictions following from the overall organization of the FDG model are supported by empirical data; and (ii) to refine, where necessary, the classifications of modal adverbs proposed in the literature.