This paper suggests that wh-arguments such as ``who`` and ``what`` are DP-arguments, whereas a wh-adjunct ``why`` is an NP-adverbial (introduced by a null preposition). Under this proposal, it will be claimed that wh-arguments can check the u[Wh] within the DP without movement due to the presence of the head D bearing the [Q], whereas a wh-adjunct ``why`` must move to check its u[Wh] due to lack of the D. This claim will be illustrated by the asymmetry arising between wh-arguments and wh-adjunct ``why`` in the Islands in Korean/Japanese, supporting that the Phase Impenetrability Condition constrains covert wh-movement as well as overt wh-movement.