to hit someone with a vehicle and injure or kill them

She was knocked down by a bus. 她被一辆公共汽车撞倒了。

to cause someone or something to fall to the ground by hitting him, her, or it

to reduce a price, or to persuade someone to reduce the price of something they are selling

She wanted $200 but I knocked her down to $175. 她开价200美元，我还到175美元。

to destroy a building or part of a building

There are plans to knock the library down and replace it with a hotel complex. 计划拆除图书馆，改建酒店综合大楼。

to show that what someone has said is not true

She easily knocked down every argument he put up. 她轻而易举地驳倒了他提出的每个论点。