1. Difficult（最常用）
例句：Learning a new language can be difficult.
搭配：difficult task/problem/situation
2. Hard（更口语化）
例句：This math problem is too hard for me.
注意：hard 本身也有"坚硬的"含义
3. Challenging（隐含"需要克服的挑战"）
例句：The marathon was physically challenging.
4. Tough（口语，多指处境艰难）
例句：It's tough to find a job in this economy.
【扩展词汇】
Arduous（强调需要持续努力）: an arduous journey
Demanding（高要求）: a demanding job
Formidable（令人敬畏的困难）: a formidable opponent
Complex（复杂导致的困难）: a complex issue
Laborious（费时费力）: laborious paperwork
提示：日常交流中前4个最常用，学术写作中可适当使用扩展词汇。注意形容词与名词的搭配习惯，比如：
have difficulty (in) doing sth.
face challenges
encounter obstacles
