那是个很难的问题。

That's a very hard question.

《柯林斯英汉双解大词典》

很难给这个问题确切定性。

It's difficult to define the exact nature of the problem.

《牛津词典》

对初学者解释这个问题很难。

It was difficult to explain the problem to beginners.

《牛津词典》

I'm not going to have you work long hours on difficult problems to be submitted and then graded.

我的作业不会让你们花费大量的时间,去钻研很难的问题，然后交上来让我打分。

麻省理工公开课 - 固态化学导论课程节选

Part of the problem with using operant conditioning with babies is it's difficult to get them to behave in any systematic way.

应用操作性条件作用的部分问题在于,很难使婴儿们按照系统的方式行动

耶鲁公开课 - 心理学导论课程节选

For a couple of reasons. In some ways, this would be nicer, do expected cases, it's going to tell you on average how much you expect to take, but it tends to be hard to compute, because to compute that, you have to know a distribution on input.

关注最快的情况，在某种意义上来说，因为一些原因这样想挺不错的，当我们处理一个给定的问题,计算平均时间的时候,是很难计算的，因为你并不知道输入的分布情况,这些输入会是怎么样的呢？

麻省理工公开课 - 计算机科学及编程导论课程节选