请问下面句子中的 were to play jokes 是什么使用方法，这个不定式是表语吗？请老师剖析一下，谢谢！
However, there was one band that started in a different way. It was called the Monkees and began as a TV show. The musicians were to play jokes on each other as well as play music, most of which was based loosely on the Beatles.
句子来源：高中英语人教版必学二第五单元（Unit 5）。
▲你问的句子，是表示以后的“be+动词不定式”结构，不是“系表结构”。
关于这个问题，通常语法书都有论述，但大多不够详细。
▲在不一样的语境中，“be+动词不定式”结构可以表示多种不一样的语义。
1、表示“计划”或“安排”。
This line is to be opened to traffic next week. 该线路将在下周通车。
They are to meet at the gate of the school. 他们将在学校门口见面。
2、表示“命令”。通常译为“需要”或“不准”。
No one is to leave the room without permission.
未经允许哪个也不准离开房间。
This naughty boy is to stay here until class is over.
这个俏皮的小孩需要待在这里，一直到下课为止。（老师惩罚他）
3、表示“可能性”。
Are you to pay the debt for your wife?
你能为老婆还债吗？
The lovely puppy is nowhere to be found.
这只可爱的小狗恐怕哪儿也找不到了。
4、表示“意图”或“计划”。通常译为“想要”。
If you are to be there on time, you had better hurry.
你想要按时到达那儿，最好快点走。
If all of us are to remain friendly, we must be sincere with one another.
假若大家都想维持友好相处，大家需要以诚相待。
5、表示“应该”。
You have done so well. You are to be rewarded.
Such nasty questions are to be avoided.
6、表示“不可防止”或“注定发生”。比如：
Your plan is to be a failure.
The murderer was sentenced to life imprisonment and was never to see his family again.
She did not think too much of her unkind acts. However, they were to be harmful to herself, too.
Tom wasnt worried about his head injury, but it was to be troublesome for him later.
7、表示“期待”。
The actual use of the newly invented equipment is still to be determined.
Henrys appointment with the firm is yet to be /confirm/ied.
8、表示与以后不符或相反的“假设”。
If it were to rain tomorrow, we would not go outing.
Even if you were to take a taxi now, you would not be able to arrive at your destination on time.
▲关于你提问的句子The musicians were to play jokes on each other as well as play music中，were to play jokes 应该理解为第六种使用方法，“不可防止做某事”。
