It is posited in this paper that in writing, paralleling the process of oral communication, ”description sufficiency” as manifested in structural complexity, is a premis. The teacher of the Advanced Writing class in the present action study aimed to facilitate syntax learning and explicitly instruct the 19 university juniors to learn the seven types of generative post-nominal constituents in English, i.e., prepositional phrases, infinitive phrases, participle phrases, adjective clauses, adjective phrases, noun phrases, and noun clauses. Thereafter, the differences in the target categories of the syntactic complexity found in the writing assignments of the learners between the pre-learning narrative and post-learning expository sets of data were compared and contrasted qualitatively and quantitatively.The research results indicated that the learners began acquiring the six out of seven types of modifiers and applying them in their expository writings. In addition, the most preferred structure statistically remained relative clauses, which stayed problematic and needed further tackling. For both narrative and expository writings, subject-verb agreement in number and tense in the relative clauses fell into the largest category of errors, which revealed L1 interference and also needed further instruction.