“No student,”the headteacher stressed, “_______ go out of the school after ten o’clock at night without my permission. ”

The children _____lost in the woods; otherwise, they would have been at the lakeside camp as scheduled. 

---Where is my pen?--- I _____it.

—Must I go with you?—No, you ________.

--- Listen! David is talking happily with his mother in the next room.　--- It ______ be his mother. She has gone to Yunnan to be a volunteer doctor.