You understand me never regretful, be like don't understand a night in the daytime of black.
你永远不懂我伤悲。就像白天不懂夜的黑。

Then my grandpa died after a couple of days. I am so regretful that I can never forgive myself.
没过几天爷爷就去世了, 我真是后悔莫及。

Regretful is academia to public policy benefit relevant theory studies achievement is very little.
遗憾的是学术界对公共政策效益相关的理论研究成果甚少。

All of a sudden, he realized that he had hurt her, so he is deeply regretful for what he has done.
一瞬间，他意识到自己伤害了她，很后悔。

Don't idle away your time when you are young, otherwise you will feel sad and regretful when you are old.
莫等闲，白了少年头，空悲切！

He felt deeply regretful when he was arrested by the police, but it was too late for him to mend his ways.
警察抓他时他非常后悔，但已是江心补漏，为时已晚。

The dress takes to loosen gradually and I am more and more emaciated, No regretful plying at all, I am rather for her only distressed as I did.
衣带渐宽终不悔, 为伊消得人憔悴。

The dress takes to loosen gradually and I am more and more emaciated, no regretful plying at all, I am rather for her only distressed as I did.
衣带渐宽终不悔，为伊消得人憔悴。

He also knows to think before he leaps so that he will not feel regretful afterwards. Moreover, he knows to tolerate others, so he can naturally lead a happy life.
他明白事不三思终会后悔，能够忍让，自然幸福而无忧。