I'm so sad.                                                                         

我好难过。

I was so sad that I couldn't stop my tears.                          

我难过到眼泪流不停。

I was so sad that I cried a lot.                                             

我好难过，所以我一直哭。

I'm sorry that I made you sad.                                 

抱歉，我让你伤心了。

You look sad. What happened at school?

你看起来很伤心。在学校发生了什么事呢？

I'm sad because my friend has moved to another school.

我难过是因为我的朋友转学了。

When you scold me, I'm very sad.                                      

当你责骂我的时候，我很伤心。

When you get angry, I'm sad.

当你生气时，我很难过。