自愿：adv. have to do sth. eg: 明天开始自愿上晚自习；开学前自愿捐献8000元钱。

补课：v. Let the students in China have some classes on weekends or vacations. Sometimes their parents need to hand in some money. Usually, they have to go to school. If they don't do that, their parents and teachers will criticize him.