1·How to improve the quality of products was a difficult problem to him.
如何提高产品质量对他来说可是个难题。
2·She is working at a difficult problem in mathematics.
她在做一道数学难题。
—— 《新英汉大辞典》
3·Even though these economies are only just starting to feel the upturn, policymakers now face a difficult problem: how to sustain a robust recovery without blowing up bubbles.
尽管这些经济体的经济刚开始好转，但政策制定者现在面临难题：在避免经济泡沫的情况下如何保持经济的强劲反弹。
4·He soon turned his attention to the difficult problem.
他很快把注意力转到这个难题上。
5·Providing dynamic internationalization of information for display is a difficult problem.
为显示提供信息的动态国际化是一个难题。