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 a most difficult question. 这是一个非常难的问题。
2. It was difficult to calm my brother down. 要使我弟弟平静下来是很困难的。
3. Among the problems we have to deal with, improving education is probably the most difficult. 在我们必须处理的（诸多）问题之中，改善教育大概是最困难的。
英英解释
adj.
1. not easy; requiring great physical or mental effort to accomplish or comprehend or endure
2. requiring much effort and trouble
3. difficult to manage or control
