Study Gongbuhada : basic form of the verb Gongbuhanda : the present tense of the verb Gongbuhamnida : formal＆written Korean style Gongbuhaeyo : informal＆spoken Korean style I don't feel familiar with the formal style. So it's better to talk in the informal style(Gongbuhaeyo) when you talk with Korean friends. I have no clue about -i if you didn't mean the postposition used after nouns end with one or double consonant. Could you elaborate a bit more?