My most devoted friend, ProfessorYang, _______ often go hiking when he lived in the mountain that was coveredwith forest.

You’d better ask Mr Wang for help when _______ problems.—I see, but I’m not sure when _______ time.

We have been told that under no circumstances ___ the telephone in the office for personal affairs.

It is when the plane ______ that you’d better find out at the booking office

Look at the sign ! Trucks are not allowed to go the city center .