This paper clarifies the usage and function of the phrase ‘eul jul alda’ in Modern Korean. ‘eul jul alda’ has two meanings： it can be used to express both “capability” and “non-capability.” For example, the sentence－ “naneun hangugmaleul hal jul al-ass-eo-yo” has two meanings. One meaning is ‘I could speak Korean.’ The second meaning is ‘I thought you could speak Korean [but you cannot]. ’ Discerning which of these meanings is intended in a given context does not pose a problem for Korean native speakers. However distinguishing meaning is difficult for many Japanese students of Korean.－so they may be reluctant to use the ‘eul jul alda’ form in a sentence. Methods for students to distinguish meaning as written in existing grammar book contain many obscure points, and more concrete requirements need to be developed to clarify usage. In this paper, therefore, concrete usages of ‘eul jul alda’ are provided and the relationship between meaning and usage from the syntagmatic field. The difference in meaning between “capability” and “non-capability” was found to depend on the following factors：the independent existence of an embedded sentential subject and a matrix sentential subject, the use of an imperative sentence, and the use of the past tense. In light of this, seven concrete requirements to distinguish between meanings of ‘eul jul alda’ shall be shown in this paper.