Although it is well-known that gou 苟, ruo 若, and shi 使 in Pre-Qin Chinese marked protases of conditional sentences, it has rarely been mentioned that they differ in their expressive meanings. The present paper examines the origins of these morphemes and their semantic extensions in order to account for their functional distributions. The paper conc1udes that ruo 若marks a relative or uncertain premise; gou 苟, an absolute premise; and shi 使, a premise for non-factual conditionals. Moreover, these protasis markers are not obligatory for conditional sentences, and the paper suggests that they are used only when the speaker intends to make explicit his/her attitude of expression, i.e., subjectivity of utterance.