This is the 5216th most frequent Japanese word.
難く
The Japanese word "難く" (かたく, kataku) means "hard to" or "difficult to."
難く is used as part of the structure 解き難い, meaning 'difficult to solve', transforming the verb 解く (to solve) into an adjective describing difficulty.
その問題は解き難いです。
That problem is difficult to solve.
難く is used as part of the structure 理解し難い, where it describes the difficulty in understanding the explanation, combining 理解する (to understand) with 難い.
彼の説明は理解し難かった。
His explanation was hard to understand.
難く is used within the context 受け難い, referring to the challenge of accepting the proposal, turning the verb 受ける (to accept) into an adjective form expressing difficulty.