語言中的情態詞主要用以表達說話者主觀的語氣及觀點，日常生活的言談語句中更是眾多情態詞百花齊放的舞台，傳達各式各樣的訊息。然而，情態詞的種類之繁雜程度卻讓傳統語法的分類及分析難以望其項背。本文聚焦「應該」與「可能」兩詞，並討論其語意呈現上的差異及句法分布上的限制，主張「應該」具有示證性及義務性語意，而與「可能」的認知性語意略有不同。藉由不同的語意詮釋，嘗試將「應該」與「可能」兩詞區分並再細分類，以符合實際語言事實，並對比各細類情態詞的分布情形及句法限制，進一步劃分界線。對於中文的情態詞而言，語意對比的呈現與句法階層的位置上，強化了製圖理論（Cartographic Approach）對語言共同性的看法。

Modals in a language convey speaker’s attitudes and viewpoints. Conversations in daily life provide a stage for modals to express various kinds of information. The various types of modals, however, have defied classifications and analyses of modals. Focusing on yinggai and keneng in Mandarin Chinese, this article mainly deals with the difference of their semantic interpretations and the constraints on their syntactic distributions. It is claimed that, with either evidential or deontic reading, yinggai should be separated from keneng, which belongs to epistemic modality. By their various interpretations and distributional constraints, these two modals are divided and sub-classified. The interface between semantic readings and syntactic hierarchy of these modals positively reinforces the concept of generality among languages proposed in Cartographic Approach.