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difficult; hard
「難い」 has other readings with different meanings.
この問題の原因を突き止めるのは難い。
It is difficult to pinpoint the cause of this problem.
彼の行動を理解するのは難かった。
It was difficult to understand his behavior.
経験者にとってこの作業は難くない。
This task is not difficult for an experienced person.