The goal of this study is to investigate the use of the two English synonymous adjectives difficult and hard through BNC, and to examine Korean college students’knowledge of difficult/hard+noun expressions. The main findings of this paper are as follows. First, The adjective difficult is mainly (83.33%) used to mean ‘not easy to do or deal with’, while hard is mainly (60%) used as idiomatic expressions. Second, both adjectives have different meanings according to the nouns which they are combined with (hard rock/life, difficult decision/concept), and sometimes they have a radically different meaning even when they are combined with the same noun (difficult man, hard man). Third, The average score of the subjects was as low as 54.50%. Fourth, The average score by type is in the order of hard+noun (77.19%), difficult+noun (45.46%) and difficult-hard+noun(38.34%), which shows that the subjects’ overall knowledge of difficult/hard+noun combinations is considerably poor and unbalanced. This result implies that Korean learners of English need to study English, not just memorizing individual words, but with a focus on chunks.