---My Dad began to teach me English when I was eight.---Wow. You must have studied English for six years, ?
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.
When I was a child, I ________ watch TV whenever I wanted to.