1. -가

    1. '-가' marks the suject of a verb or descriptive verb. It is often omitted in conversation.
    2. '-가'follows nouns which end with a vowel.
      학교가 멀어요 ? Is the school far ?
      학교가 멀어요. The school is far.
      가게가 가까워요 ? Is the shop nearby ?
      가게가 가까워요. The shop is nearby.

  2. -이

    1. The meaning of '-이' is exactly the same as '-가'.
    2. '-이' follows nouns which end with a consonant.
      그 집이 좋아요 ? Is that house good ?
      그 집이 좋아요. That house is good.
      옷이 싸요 ? Are the clothes cheap ?
      옷이 싸요. The clothes are cheap.

  3. -이

    1. Subject markers are optional in conversation.
    2. Subject markers can be replaced with a short pause.
      멀어요. It is far.
      아주 멀어요. It is very far.
      대사관 아주 멀어요. The embassy is very far.
      대사관이 아주 멀어요. The embassy is very far.