難題的英文：
a difficult problem; an arduous problem; a tough question
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。
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。
adj. 费力的，辛勤的，险峻的
Success is the fruit of arduous striving.
成功是奋斗的结果。
They are very glad that they have been assigned the most arduous task .
他们很高兴被派给最艰巨的任务。
His timidity disinclined him from such an arduous enterprise
他的怯懦使他不愿从事一项如此艰巨的事业。