问题描述：
改为被动语态
最佳答案： They built a new school last year.
改为：A new school was built by them last year.
被动语态的结构分类:
一般现在时：be done（be随人称变化）
一般过去时：was/were done
一般将来时：will be done
现在进行时：be being done（前面的be随人称变化，后面的being固定）
过去进行时：was/were being done（being固定）
将来进行时：will be being done（being固定）
现在完成时：have/has been done
过去完成时：had been done
将来完成时：will have been done
将来完成进行时：will have been done
现在完成进行时：have been being done
过去将来时：would be done
过去将来完成时：would have been done
过去完成进行时：had been being done
过去将来进行时：would be being done
过去将来完成进行时：would have been being done
