I don’t remember how many years ago _____ I was taken to Beijing for the first time.

These books______very interesting. Many children like reading them.

Tommy, do you know if Frank __ to the zoo this Sunday if it __?–Sorry, I have no idea.

Why didn’t David ________his friend’s invitation to the picnic?—Because he had to prepare for the exam.

What do you think of math? — I find difficult to learn it.