—Have you been wasting time on computer games again? — ________ . I’ve been studying a lot and I need a break.
It ________be the postman at the door.It's only six o'clock.
It ________ be the vocabulary that caused you the problem in the exercise because you know a lot of words.
—What are you doing this Saturday?—I'm not sure, but I ________ go to the Rolling Stones concert.
I'm going to Europe on vacation together with John if I ________ find the money.
He ________ sleep, although he tried to, when he got on such a hunt for an idea until he had caught it.