Sit down.

坐下！

I'm writing to a friend of mine in South American.

我在写信给一个美国南部的朋友。

He answered almost all of my questions.

他回答了我几乎所有的问题。

How did you break your leg?

你怎么弄断了腿？

According to Mr. Green,this is a complicated problem.

对于格林先生，这是一个复杂的问题。

How long has it been since you've heard from your uncle?

自从你听到叔叔的消息有多长时间了？