difficult 比hard较为正式，侧重需要特别的能力、智力、判断力、技巧或勇气才能克服障碍。也可指深奥或抽象的难题。
hard 含义广，通俗用词，与easy相对，泛指任何难理解，不好处理或不易做的事。
参考例句
1. It is hard for me to keep silence. 让我保持安静对我来说是很难的。
2. To sum up, success results from hard work. 总而言之，成功是艰苦努力的结果。
3. We need to work hard to make a perfect world. 我们要努力工作以创造一个完美的世界。
英英解释
adj.
1. not easy; requiring great physical or mental effort to accomplish or comprehend or endure
2. dispassionate
3. resisting weight or pressure
4. very strong or vigorous
5. characterized by toilsome effort to the point of exhaustion; especially physical effort
6. produced without vibration of the vocal cords
7. (of light) transmitted directly from a pointed light source
8. (of speech sounds); produced with the back of the tongue raised toward or touching the velum
9. given to excessive indulgence of bodily appetites especially for intoxicating liquors
10. being distilled rather than fermented; having a high alcoholic content
11. unfortunate or hard to bear
12. dried out adv.
1. with effort or force or vigor
2. with firmness
3. earnestly or intently
4. causing great damage or hardship
5. slowly and with difficulty
6. indulging excessively
7. into a solid condition
8. very near or close in space or time
9. with pain or distress or bitterness
10. to the full extent possible; all the way
