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
/silentwolfyh/article/details/95380086
二、被动语态与情态动词
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
三、 推测过去的被动 （must have been done）
Over a hundred peoplemust have been drivenawayfrom their homes by the noise.
My cell phonemust have been stolen.
His keysmust have been lost
