from one to five yesterday The boy jumped up and down.这男孩跳了一下The boy was jumping up and down.这男孩不停地跳着jump up是跳 为什么要加一个down

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from one to five yesterday
The boy jumped up and down.
这男孩跳了一下
The boy was jumping up and down.
这男孩不停地跳着
jump up是跳 为什么要加一个down

jump up是跳没错,但是跳了总要下来的吧,这是人家说的习惯,就像我们说的:我来了.你却没见人只说:我来