用所给动词的正确形式填空。 1. Did you see the accident ______ this morning? (happen) 2. You should get a chair ______on. (sit) 3. There are so many children ______ (play) in the park on Sundays. 4. It's hot. You'd better ______ both the windows and the doors. (open) 5. It's not easy ______ fish with your hands only.(catch) 6. Our teacher asked us to ______ page 16. (turn) 7. You must ______ to the teacher carefully in class. (listen) 8. He felt ______ (worry) about his son at that time. 9. He found a number of people already ______ (work) in the workplace. 10. Many children trust animals because they know ______ (keep) a secret.