一、
work hard: 此时work是动词，hard是副词，副词修饰动词；hard work: 此时work是名词，hard是形容词，形容词修饰名词。
二、
hard work 指的是 艰苦困难的工作
work hard 指的是 工作努力
hard 的词性不同,上面是形容词adj,下面是副词adv
拓展内容：
work hard 例句：
I work hard to keep that trust.
我为了保持这种信任努力工作。
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It would advantage you to work hard.
努力工作对你会有益。
《21世纪大英汉词典》
I understand that all of us want our close friends and family to be proud of us so we work hard inschool or in our careers to meet their expectations.
我理解所有人都想让亲朋好友，家人为我们而感到骄傲，为了达到他们的期望所以我们在学校努力学习，在职场上努力工作。
hard work 例句：
He ascribed his success to hard work.
他把自己的成就归功于努力工作。
《21世纪大英汉词典》
He has won many young people to the Party by his hard work.
他通过努力工作争取了很多年轻人加入这个党。
《21世纪大英汉词典》
After years of hard work, she has finally arrived in her field.
经过多年的努力工作之后她终于在她的领域功成名就了。
《21世纪大英汉词典》