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.
Hey, what's the matter? You look down.
She's been really down since she left home.
Down adj. 系统停止运转
The system was down all morning.
Down payment 首付款
I am saving for a down payment on a house.
Be down to sb 由某人负责
After his father retires, it will be down to him to lead the company.
Upside down 上下颠倒的(地)
Turn the bottle upside down and shake it.
Let sb. down 让某人失望，辜负某人
I was counting on John to come, but he let me down.