In this research, I intend to examine the context functions of non-assertive expressions, "darou, no-darou, darouka, no-darouka," from the aspect of Pragmatics. Throughout data-collection and analysis of usages in real conversational scenes, synonymous expressions "(no) darou (ka)" could be classified into 6 varieties: affair speculation, condition speculation, question, query, irony, strong irony. And the analysis result shows that the two factors determining the differences between "darou (ka)" and "nodarou (ka)" are "explanation" and "emphasis" of "noda."In addition, to evaluate the acquisition conditions of Taiwanese JFL learners and their differences from that of native Japanese speakers, I have conducted a survey of both JFL learners and Japanese native speakers. According to the survey, I found out the three results as below:1. JFL learners tend to use unnatural expression "darou" rather than natural expression such as "(no)kana" or "(n)janai" when they attempt to make a conjecture in conversations.2. JFL learners cannot distinguish between "darou" of "affair speculation" and "nodarou" of "condition speculation" as well as "darouka" of "question" and "nodarouka" of "query."3. JFL learners are not aware that the interrogative "ka" can be omitted in the functions of "question" and "query."