3 sentences

1. 他后悔在校时没用功读书。

   tā hòu​huǐ zài xiào shí méi yòng​gōng dú​shū.

   He regrets not having worked harder at school.

   Source: Tatoeba

2. 这孩子挺聪明，就是吃亏在不肯用功。

   zhèi hái​zi tǐng cōng​ming, jiù shì chī​kuī zài bù kěn yòng​gōng.

   The child is quite smart, but his drawback is that he won't work hard.

   Source: Dong Chinese Dictionary

3. 他是个非常用功的学生，每天都早起复习功课。

   tā shì gè fēi​cháng yòng​gōng de xué​sheng, měi tiān dōu zǎo​qǐ fù​xí gōng​kè.

   He is a very hardworking student who gets up early every day to review his lessons.

   Source: Dong Chinese Dictionary