What is the different between work hard and work harder

work hard 是”努力工作“的意思，

work harder 也是“努力工作”的意思，不過work harder有個比較的概念

比如, Jennifer works harder and is more careful than Jane.

in general,both means that someone should be more industrious.
but there is a comparison

e.g you should work hard
-----it implies that u are too lazy
e.g you should work harder
----it implies u are not too lazy,but u shouldl work harder to achieve good results

hope it can help you

word hard (努力地工作)

e.g. He gets good results in examinations because he works really hard every day.
因為他每天努力工作，所以在考試中取得好成績。

work harder(更加努力地工作)

work harder 與 work hard 多了一重比較(comparison)的意味。

e.g. Johnny works harder than David.
莊尼比大衛更加努力地工作。

e.g. 'You should work harder and get a better result next time!' said the teacher.
老師說：「你應該要更加努力地工作，在下次的考試中取得一個更好的成績！」