the musicians were to play jokes on each other 这句中为什么要加介词to?

the musicians were to play jokes on each other 这句中为什么要加介词to?
the musicians were to play jokes on each other 这句中为什么要加介词to?

the musicians were to play jokes on each other 这句中为什么要加介词to?
was/were + to 过去将来时
am/is /are + to do sth一般将来时,表示按计划,约定将要发生的事情
eg:we are to meet at the school gate tomorrow morning.
was/were +to do sth 过去将来时,表示在过去的时间来看,事情将会发生.使用这种时态时,要注意时间的参照点.在间接引语中用的比较多.
eg:He said he was to leave for Beijing next week.

这个to个人理解是动词不定式做表语，表示目的，表示“是为了互相开玩笑”。
希望看到上下文，这样可以比较容易做出更准确的判断。