be to do:
(1)表示事先商定、安排或准备要做的事情.如：
The students are to meet at the school gate tomorrow.明天学生们将在学校大门口集会.
2）表示可能性,必要、责任、义务、禁止等.如：
Her necklace was not to(couldn't)be found.她的项链找不到了