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Before you     the room, you should knock at the door first.

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Hit  Knock  Touch  Strike  
of turn off    have turned off    that you turn off    turning off  
take off  put off  get off  turn off  
Knock    Knocking    Knocked    To knock  
to knock    knocking    knocked    knock  
to turn    having been turned    to have turned    turning  
make sure      be sure    bring out      hold up  
must; should    can; shall    must; shall    can; should  
choose    measure    build    discover  
must; should    can; shall    must; shall    can; could  
when    if    while    before