Down adj. 不高兴的；情绪低落的

unhappy; unable to feel excited or energetic about anything

I feel a bit down today.

我今天心情不太好。

Hey, what's the matter? You look down.

喂，怎么了？你看起来不高兴。

She's been really down since she left home.

自从离开家后，她就一直闷闷不乐。

Down adj. 系统停止运转

The system was down all morning.

整个早上系统都是坏掉的。

Down payment 首付款

I am saving for a down payment on a house.

我正在攒钱付首付买房子。

Be down to sb 由某人负责

After his father retires, it will be down to him to lead the company.

他的父亲退休后，将由他来领导公司。

Upside down 上下颠倒的(地)

Turn the bottle upside down and shake it.

我瓶子翻转过来，摇一摇。

Let sb. down 让某人失望，辜负某人

I was counting on John to come, but he let me down.

我指望着John来呢，但她让我失望了。