1. 首先down有降低的意思比如：knock down: 降价、下跌：
The market might abandon the stock, and knock down the price.
2. knock down还有倒下的意思：
He was knocked down by a car.
The suspect was gunned down by the police.
I turned down the job offer.
The computer breaks down occasionally. We need to buy new ones.
I need to shut down the computer and take a nap.
5. down： 心情压抑
The weather is getting me down.
He looks down on her and doesn't show any respect to her.
I cannot believe that she puts him down like that.