I should have studied hard.

我当时应该用功读书的。

I should have listened to Mom.

我当时应该听妈妈的话的。

I shouldn't have bought this toy.

我当时不应该买这个玩具的。

I'm so sorry that I didn't listen to you, Mom.

妈，我很抱歉没有听你的话。

It's no use being sorry now.

现在后悔也没用了。

Study hard now, or you will feel sorry later.

现在要用功读书，不然你以后会后悔的。