1. It's hard/difficult to do something
最基础表达
例句：It's hard to learn a new language at an older age.
例句：It can be difficult to concentrate in a noisy environment.
2. Have trouble/difficulty (in) doing something
强调过程中遇到困难
例句：She had difficulty understanding the local accent.
注意：后接动名词（doing），介词in可省略
3. Struggle to do something
暗示需要付出持续努力
例句：Many students struggle to meet the deadline.
4. Find it challenging to do something
较正式的表达
例句：Researchers find it challenging to replicate these results.
5. It's no easy task to...
强调事情本身难度大
例句：It's no easy task to climb Mount Everest.
6. Be not a walk in the park
口语化表达
例句：Writing a thesis is not a walk in the park.
注意事项：
"Hard" 较口语，"difficult" 更正式
"Struggle" 后可接 with + 名词（如：struggle with math）
否定可用 "have no trouble doing..." 表示顺利
建议根据具体语境选择：
学术写作常用：pose challenges, prove difficult
日常对话多用：tough, tricky（如：It's tough to get up early）
