28. Dr. Frank failed many times but he finally _____ to find a successful way to solve the problem.
Everyone _____ do his best for the modernizations of our country.
31. He _____ have come here yesterday, but he didn’t.
He ought to win the first prize, _____ he?
I missed the last bus, so I _____ go home on foot.
I got up early that morning, but I _____ so because I had no work to do.