Abstract:          Input is the initial stage of L2 acquisition, and the difficulty of input directly impacts on the comprehensibility of input.Based on Markedness Hypothesis and with the method of timed Grammatical Judgement Test, this paper investigates the input  difficulty of Cai and Jiu as related to their semantic meanings in expressing "time" and "relations". Results show that unmarked Jiu  has less input difficulty than marked Cai, and at the same time the L2 acquisition of these two adverbs are influenced. Cai and Jiuh as no difference in the comprehensibility when expressing the concept of time, but are significantly different when expressing relations. As   regards to the development of L2 knowledge, acquisition of Jiu is better than that of Cai because of the better input comprehensibility.
  

Key words: input, comprehensibility, L2 Chinese, Cai, Jiu