All the students in the class except the sick boy (were encouraged) to plant trees yesterday.
问题描述：
All the students in the class except the sick boy (were encouraged) to plant trees yesterday.
为什么用(were encouraged)
答
这句话的主语是All the students,是复数形式；然后“这些学生被鼓励去植树”是昨天（yesterday）的事情,所以用过去式.
这样就只能用were encouraged了.