keep somebody back（宾语在back前面）阻挡，拦阻，制止其前进：
Frequent illnesses kept him back.
What is the best way to keep down rats？（keep them down）
Try to remember to turn off the light when you leave the room.I am trying to keep down expenses.（keep expenses down）
‘Keep off the grass.'
keep on 继续，不停歇：
I wanted to explain but he kept on talking and didn't give me a chance to say anything.
My shoes are very old and don't keep out the water.（keep the water out/ keep it out）
He began walking at four miles an hour but he couldn't keep up that speed and soon began to walk more slowly.（couldn't keep it up）
It is difficult to keep up a conversation with someone who only says‘Yes'and‘No'.
keep up（intr），keep up with（tr）与某人齐头并进，和某人同速前进：
A runner can't keep up with a cyclist.
The work that the class is doing is too difficult for me.I won't be able to keep up（with them）.
It is impossible to keep up with the news unless you read the newspapers.
English workmen usually knock off at 5.30 or 6.00 p.m.
We knock off work in time for tea.
In the finals of the boxing championship he knocked out his opponent，who was carried out of the ring.（knocked his opponent out/knocked himout）