N 후에, V-(으)ㄴ 후에: This pattern means 'after a certain period of time' or 'after some action. Somewhat like 'after' or 'later' in English. V-고 나서: This expresses the finishing of one behaviour followed by the beginning of a subsequent behaviour and hence corresponds to 'do (something) after', 'upon finishing' or 'and then' in English. Because -고 나서 expresses temporal order, it can only be used with verbs. If the subject of the first and second clauses are the same in sentences using motion verbs (e.g. 가다, 오다), -아/어서 is used instead. e.g. 나는 학교에 가서 공부해요. (and not 나는 학교에 가고 나서 공부해요). V-아/어서: a connective ending indicating the temporal relationship between events. The action of the second clause occurs after the first clause has occurred. The two actions are so closely related that the second action cannot occur without the first action occurring first. Somewhat like 'and' or '(in order) to' in English. Source: Korean Grammar in Use Beginning