This study examines the fundamental rule of division between Thai Interrogative pronoun and Indefinite pronoun which is the same words. Special attention will be given to the morphological characteristics of classification rule in both cognate pronoun, interrogative pronoun and Indefinite pronoun. Based on the observation of word arrangement position in general and appending words in Particular, this paper claims that there are clear differences in the form of expressing one``s intention between two pronouns. Based upon the views of sociolinguistics and ecolinguistics, this paper suggests that the problems of endangered languages should be more widely understand in the field of foreign language education.