This paper attempts to distinguish "Chadianr (mei)" from the perspective of the discourse, found that "Chadianr (mei)" is selected by the constraints of Context: if the events in the sentence have the formation of necessity causality, you can only use affirmative meaning "Chadianr mei" emphasizes tone; if they have contingent causality or the action is impossible, then use the "Chadianr" and the negative meaning "Chadianr mei" emphasizes tone; " Xiwang" or not only affects "Chadianr (mei)" in different pragmatics meanings, not the selection criteria of "Chadianr (mei)". In addition, through the corpus statistical analysis found that "Chadianr" has highest frequency of use; "Chadianr mei" has very low frequency, and especially the affirmative meaning "Chadianr mei" has the lowest frequency. Therefore, the "Chadianr" and "Chadianr mei" should be taught in different phases, in order to avoid interference between the same shape isomer.