From “Categories of the Theory of Grammar” (Halliday in collected works of M. A. K. Halliday. Continuum, London, pp. 37–94, 1961) to Introduction to Functional Grammar (Halliday in An introduction to functional grammar. Edward Arnold, London, 1985; An introduction to functional grammar, 2nd edn. Edward Arnold, London, 1994; Halliday and Matthiessen in An introduction to functional grammar, 3rd edn. Edward Arnold, London, 2004, Halliday’s introduction to functional grammar, 4th edn. Routledge, London, 2014), SFL created by Halliday has developed into a general linguistics from a grammar theory and has become a mainstream linguistics in the world (Fawcett in Theory of syntax for systemic functional linguistics. John Benjamins, Philadelphia, 2000; Eggins in An introduction to systemic functional linguistics, 2nd edn. Continuum, London, 2004). In “Categories of the Theory of Grammar” (Halliday in collected works of M. A. K. Halliday. Continuum, London, pp. 37–94, 1961), Halliday outlined four categories of language: unit, structure, class, and system. Later, in “Some Notes on ‘Deep’ Grammar” (Halliday in J Linguist, 2:57–67, 1966), he focused on the two categories: system and structure. From the system category, he developed systemic grammar, and from the structure category, he developed functional grammar. In systemic grammar, language is considered as a meaning system, and form is the realization of meaning. However, without a formal theory, the meaning will not be realized. In functional grammar, form is represented as the constituent structure of rank, based on which the functional syntactic structures related to the three metafunctions are established, including the transitivity structure realizing ideational metafunction, the mood structure realizing interpersonal metafunction, and the thematic and information structures realizing textual metafunction. Therefore, “syntax plays a very important part, even a central part in SFL theory” (Huang in Foreign Lang Res, 3:39–45, 2007).