The work deals with cognitive synonymy studied on the example of two lexical items - the adjectives "difficult" and "hard" - which are regarded to be related by this kind of synonymic relation. The characteristic feature of the lexical items analyzed in the work is their interchangeability in some contexts; however, it is often the case that due to the existing differences in the meaning of synonymous expressions, this substitutability is not possible in some contexts. The work thus shows in what aspects "difficult" and "hard" differ and what their collocational restrictions are. The book is divided into theoretical and practical parts. The theoretical part provides insight into necessary theoretical background; a particular attention is paid to the notions of "meaning" and "synonymy" and to the theory of collocations. Moreover, the work emphasizes the importance of corpora and introduces the British National Corpus (BNC) on which the research on synonymity is based. The practical part presents the actual comparative analysis of the adjectives "difficult" and "hard". First, it offers their definitions taken from various dictionaries and then it interprets data from the BNC.