"We have not shied away from raising the difficult problems that exist and stand in the way of the people of Africa realizing their potential.

VOA: standard.2009.08.16

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

Very difficult problems, but in my judgement, all solvable if we generate the country in the kind of bipartizan political cooperation and political courage that will be necessary to fix them.

这些问题很不容易解决，但我认为,不是不能解决，只要我们,以这种两党制的合作形式治理国家,还要有治理它们的政治勇气。

普林斯顿公开课 - 国际座谈会课程节选

Political tensions and conflict have made it difficult to deal with the problems.

VOA: special.2009.05.18

The sense of values we have, the things that we get excited about are different, and the problems may be more difficult.

我们的价值体系,我们的兴趣范围都是和你们不一样的,这些问题可能会更难

耶鲁公开课 - 基础物理课程节选

He says the student competitors showed they could solve difficult technological problems.

VOA: special.2009.04.24

Now, as I said last time, the subject of free will--or free will, determinism, causation and responsibility, this cluster of problems-- is an extremely difficult and complicated physical problem.

正如我上次所说的,自由意志这个主题 或者说自由意志,决定论,因果论,责任,这一堆问题,是极端困难和复杂的物理问题

耶鲁公开课 - 死亡课程节选

Congress found it difficult to act on these problems.

VOA: special.2009.03.12

I assume that those problems are not that difficult and I've given you a lot of practice.

我知道这些题目没那么难,我已经让你们练过不少次了

耶鲁公开课 - 基础物理课程节选

Some problems are difficult to solve.

VOA: special.2009.07.26

But the president reminded an enthusiastic crowd of Democrats that solving long-entrenched problems is a difficult process.

VOA: standard.2010.02.06

"The problem with that body of research is that it's difficult to interpret because obviously people with existing mental health problems use cigarettes to self-medicate."

VOA: standard.2010.06.15

It says these problems are made even more difficult by the sweltering summer heat.

VOA: standard.2009.07.03