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

 —Who’s that boy reading in the study ? Is it David?—It __________ be David. I saw him go home just now.

He would rather _______ at home than ______ to the park with Jenny.

If you ___ stick to the unrealistic approach, the worse result will be waiting for you not far ahead.

When the traffic lights are red, we _______ stop.