I’m impressed with his determination. When he had a problem to solve, he  _____ work at it until he found an answer.

—Where did you get this book?—I picked it up from a second-hand bookshop if you ______ know.

It is important to know about the cultural differences that        cause problems.

The man over there _________ be over fifty. He looks so young.

I haven’t seen my old friends for years, I ‘m  looking  forword ______ them.