Yì yán nán jìn (一言难尽)

What does it mean?

Yì yán nán jìn means it's a long story to tell. Here yì yán means one word. Nán jìn means hard to tell. The whole phrase means it's hard to tell the whole story in one word. It's usually used while someone asks about a complicated situation.  

How to use? 

The usage of this idiom is like a sentence and an adjective. For example:

（1）A：什么？你和安娜分手了？

            Shén me? Nǐ hé Annà fēn shǒu le?

            What? You broke up with Anna? 

        B：哎，一言难尽。 

             āi, yì yán nán jìn.

             ai, it's a long story.

（2）昨天发生的事情真是一言难尽。

       Zuó tiān fā shēnɡ de shì qinɡ zhēn shì yì yán nán jìn.

       It's really a long story to tell what happened yesterday.