—Who______ it be that is knocking at the door?   —It ______ be Father, but I’m not sure. 

—It’s hard to believe that Jack should have fought with the policeman.—Yes. If one ____ ask for trouble, it can’t be helped.

English is a language that many people around the world           not speak perfectly but at least can understand．

________ stop advertising, prices would be greatly reduced.

He ________ get into his own house because he has lost his keys.