The purpose of this thesis is to resolve the ``은/는`` and ``이/가`` substitution errors committed by learners, by using ``은/는`` and ``이/가`` premises and situation models. ``은/는`` accepts the basic premise that when the subject is fixed, the predicate is selected, while ``이/가`` accepts the basic premise that when the predicate is fixed, the subject is selected. Out of the plural candidates capable of selection, the one chosen by the speaker is the important information focused upon, and the preceding studies refer to this as new information. Ultimately, what comes after is important for ``은/는``, whereas what comes before is important for ``이/가``, and thus, they may each be explained by a predicate focus or a subject focus. Based on such premises, the semantic functions of 은/는`` and ``이/가`` may be presented, which were expressed as situation models that show when they are used and in what kind of discourse situation. Because the use of 은/는`` and ``이/가`` is always connected to the discourse situation, the contextual information of the discourse situation must be given, and it must show that the postposition is selected within it. Consequently, the learner perceives more delicately and clearly the context selected as well as the difference between the two postpositions so that when writing or speaking, the learner can select and choose the adequate one between ``은/는`` and ``이/가``.