In a sentence, the grammatical function of words depend on postpositions.
철수가 공을 쳤어요 (Cheolsu hit the ball).
공이 철수를 쳤어요( The ball hit Cheolsu).
Postpositions `가/이’ come after nouns, pronouns, and numerals to make the subject form.
Proposition`을/를’ come after nouns, pronouns, and numerals to make the object form. Other postpositions consist of `에’ which signifies `direction’ or `destination,’
`에서’ which signifies `location,’ and `로/으로’ which signifies `the means’ or a `tool.’
서울에 가요. (I’m going to Seoul.)
학교에서 공부해요. (I study at school.)
택시로 가요. (I’m going by taxi.)