| 疑难 | yí nán | hard to understand / difficult to deal with / knotty / complicated |
| 盘根错节 | pán gēn cuò jié | twisted roots and intertwined joints (idiom); complicated and very tricky / knotty and deeply-rooted difficulties |
| 疑难问题 | yí nán wèn tí | knotty problem / intractable difficulty |