“Modality” is a concept used in philosophy, logic, and linguistics. Modality in linguistics is a semantic category that refers to the subjective attitude of a speaker towards the truth value of the proposition denoted by a sentence. Formally, modality is expressed using lexical or syntactic means. English, for instance, uses modal verbs like ‘may’, ‘will’ and ‘can’. Modality in Chinese also takes modal auxiliaries as its major means of expression. These auxiliaries share the following syntactic properties (Chao 1968; Zhū 1982).

First, only predicative elements can function a…