They often change the kind of crop in each field ____ few years, for example, growing corn or wheat and then the next year peas or soybeans.
The students _______ busily when Miss Brown went to get a book she _______ in the office.
I think you must be mistaken about seeing him at the theatre; I’m sure he _____ abroad all week.
They haven’t arrived yet but we _____ them at any moment.
I feel sure I _____ her before somewhere.
You’ll never guess who I met today — my old teacher! We _____ for 20 years.