Be to do sth . <1>is quite often used to refered to a future arrangement or plan,in which case it resembleds the future use of the present progressive. < 2>Aonther meaning of be to do sth . could be alternatively expressed by the modal auxiliary must in the sense of "compulsion"
Be to do sth 通常可以指将来的安排和计划.在这种情况下,类似现在进行时的表将来的用法.另一种意思是也能表示情态动词must的"强迫""命令"的意义.
1.The mayor is to visit our school tomorrow.(visiting)
2.You are to finish your homework before you can watch TV.(must)