The Reason Why Noun+dakeda Becomes Impermissible

Why is otōto wa 10-sai dakeda Impermissible?

Abstract

Learners of Japanese as a second language tend to misuse dakeda ‘is just ...', as in otōto wa 10-sai dakeda-kara nomenai "As my brother is just ten years old, he is not allowed to drink." A previous study says that dakeda is impermissible in a noun sentence when it follows a predicate nominal indicating the quality of the subject.

In this paper, I note that some examples of dakeda come after a predicate nominal indicating the quality of the subject. X+dakeda presupposes a set of X+non-X, and denies the presupposition, as in "Breakfast is just a banana."

Denying the presupposition of a set of X+non-X exists implies that there is no non-X, but only an X. However, dakeda in a question cannot presuppose a set of X+non-X. As far as dakeda following a predicate nominal indicating the quality is concerned, X+non-X becomes "The subject is X and at the same time it is non-X," which does not make complete sense. For example, dakeda in the above sentence, meaning "My brother is just ten years old," is impermissible because it is impossible to presuppose that my brother is ten years old and at the same time twenty years old.