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very difficult to ...; nearly impossible to ...
この話はなしは信じしんじがたい。
This story is hard to believe.
彼かれの言い訳いいわけは理解りかいしがたいが、聞いきいてみた。
His excuse is hard to understand, but I listened.
忘れわすれがたい思い出おもいでになった。
It became an unforgettable memory.