第一、Catch

You're giving me half a million?! What's the catch？分我五十万！The catch？

A catch is any tricky or concealed drawback to a deal.

a catch是指某件好事的附带条件。

For example, when you go on a web site, and they tell you you have a chance to win a laptop, there's usually a catch.

例如：某些网站如果说你有机会赢一台手提电脑，通常都要回答问卷，订阅周刊等等好多好多条件，所以这些条件就是a catch。

There is no catch.没有别的企图

Everything that sounds too good has a catch to it.天下没有白吃的午餐。

第二、Grand

One grand is equivalent to one thousand dollars.

one grand是一千块钱的意思。

五千块要说five grand, 因为grand代替了thousand dollars这两个词。但为什么是说five grand而不是 five grands呢？

因为grand已经是复数了, 所以要说five grand而不是five grands。