备注：21课是第10课的加强版
一、被动语态与时态相结合参考：10被动语态，双重所有格
https://blog.csdn.net/silentwolfyh/article/details/95380086
Planes are heard.
Planes can be heard.
Planes may be heard.
Planes must be heard
这封信必须马上寄出。
The letter must be sentimmediately
Over a hundred people must have been drivenaway from their homes by the noise.
My cell phone must have been stolen .
His keys must have been lost
