难以相信的是，它确实有效。

Unbelievably it actually works.

《牛津词典》

突然，难以相信的事情发生了，他拔出了手枪。

Suddenly the unthinkable happened and he drew out a gun.

《牛津词典》

让人难以相信的是，我们能够治疗更严重的疾病，却无力对抗像感冒这样常见的疾病。

It seems hard to believe that we can deal with more serious diseases yet are powerless against something so common as a cold.

youdao

Because people do all sorts of behaviors which become very, very hard to interpret if they don't really believe their bodies are going to die.

因为人们有各种行为,如果他们不相信自己的肉体会消亡,这些行为将非常难以诠释。

耶鲁公开课 - 死亡课程节选