The synonymy relationships among {하다}, {만□다}, and {크다} in Middle and Modern Korean were investigated in this study, and their historical changes were described from the ecological aspects. Their semantic similarities could be readily proved by investigating their common antonymy relationships with {젹다}, and in a sense their semantic similarities could be suggested from the meaning of their mutual permutations and corresponding Chinese characters as well. Their semantic similarities are directly shown in {하다}-{만□다} and {하다}-{크다}, but indirectly in {만□다}-{크다}. The polysemiy of {하다} in Middle Korean is appeared as two sememes, “多” and “大”, but from the viewpoint of distribution denotation, the meaning of “多” was broader than that of “大”. Describing the synonymy relationships of {하다}-{만□다} and {하다}-{크다} are also available by comparing {한} with {만□} and {큰} which are their conjugation forms respectively, and by comparing their derivative adverbs {해(하)} with {만히} and {키} which are their derivative adverbs respectively too. In Middle Korean {해} had the role of universal stative adverbs whose succeeding predicates were action verbs, but {하} had the role of degree adverbs by usually modifying stative verbs. Therefore, {많이}, stative adverb of Present Korean, is transferred into the function of degree of adverbs as {하} in Middle Korean was. Adverb {해} couldn`t have two meanings of "多“ and "大” completely, so {해} couldn`t have two meanings relationship with the derivative adverb, {키} perfectly. Because historically adjective lexeme {하다} had two meanings of “多” and “大”, it was at a disadvantage in synonymy clash competition with {만□다} and {크다} which had only one simple meaning. As a result, “多”, sememe of {하다} has been unified with {만□다} and "大” has been unified with {크게}. And also by disappearance of “ㆍ” ({□다}>{하다}) in a verb, {□다}(爲), its word form became a obsolescent word through a homonymy clash.