当约翰步行去学校时,他在一棵树上看见一只猫。When John________ ________to school,he saw a cat________a tree.
When we left, the kids said goodbye to us (礼貌).
The stamp I bought yesterday (花费)me 20 yuan.
It's a good habit to brush your (牙齿) after a meal.
People often fly kites on sunny and (多风)days.
污染如此严重,人人都应该出一份力来制止它.( so…that )