We can't _____ one to change the habits of a lifetime in a short time.

This sentence is difficult to understand.Can you make         of it?

As an official, one should serve the people at any cost _______sitting back to protect the chair．

What I want to     to teenagers is that being addicted to the Internet will to some extent do harm to both mental and physical health．

My grandfather, who          the World War Two, likes to talk to us about his experience.