1.Can I smoke here? No,you can't,it may be very dangerous.
2.I saw him at school just now, he can't be at home.
3.Must I stay at home and read somebooks at rainy days?
No,you needn't,you can watch TV.
4.What do you want to give to the teacher as present for the Teachers' Day?
I am not sure,maybe I will give him some flowers.