难题的英文：
difficult problem

参考例句：

The famous mathematician is brainiac who would rather work on solving complex mathematical problems than sleep at night.
这位著名数学家十分聪明和勤奋，宁愿晚上不睡觉，也要解开复杂的数学难题。

explain puzzles
解释难题

A tangled skein of problems
盘根错节的一堆难题

A tough game,assignment,problem,journey
艰苦的比赛、困难的任务、难题、艰辛的旅程.

luckily, the answer to this puzzle is very simple
幸亏这个难题的解答倒十分简单。

In ethics, there is a poser about the difference between killing and letting die.
就道德而论，任其死亡是否有异于杀害是一个难题。

Bill always has a card up one's sleeve, so whenever faced up with a puzzle, he could find a way.
比尔总有锦囊妙计，所以任何时候遇到难题，他总能找到方法解决。

I've just had a brainwave; I know a new approach to the difficult problem!
我刚才灵机一动,突然想出了一个解决这一难题的新办法!

Do a puzzle
解决难题

The complexities of higher mathematics
高等数学的难题.

difficult是什么意思：
adj. 困难的；执拗的；不易相处的

The exercise is that difficult!
练习题那么难！

You must admit the task to be difficult [ that the task is difficult ].
你应当承认这项任务是艰巨的。

You must admit the task to be difficult [that the task is difficult].
你必须承认这项任务是困难的。

This is as difficult a problem as you are likely to meet.
像这样的难题，你可能会碰到。

Russian is a difficult language to master.
俄语是门难以精通的语言。

problem是什么意思：
n. 难题，困难；逻辑题

It was a complex problem.
这是一个复杂的问题。

Is the problem that easy?
问题有那么简单吗?

This is a prime problem.
这是一个有关质数的问题。

These are the resolved problems.
这些是已经解决的问题。

The housing problem is bound up with the problem of gale controls.
住屋问题涉及租金控制问题。