Mathematics _______ the language of science.
If he had spent more time practicing speaking English before, he _____ able to speak it much better now.
--Why are your eyes so red? You _______ have slept well last night.---Yeah, I stayed up late writing a report.
When I was a child, I ______ watch TV whenever I wanted to.
I should not have laughed if I ______ you were serious.
He ________ sleep, although he tried to, when he got on such a hunt for an idea until he had caught it.