1、My watch ______ at nine o'clock but now it _____ .
A went stopped B was going stopped
C was going has stopped
为什么用C不用B ？
2、Now rememeber ,you ____ the text until the teacher tells you to .
A could not star B are not to star
为什么用B 不用A？

第一题,用现在完成时强调现在的一种结果,
第二题,be to 在这里表示禁止,而could表示能力,根据句意选B
另附be +动词不定式（即be + to do sth.）的用法如下：
1.表示按计划或安排要做的事。例如：
When are you to leave for home?你什么时候回家？
She is to be married next month.她将于下个月结婚。
The Queen is to visit Japan in a week’s time.女王将于一周后访问日本。
这种结构也可用于过去。was / were to do sth. 表示曾经计划要做的事，但不表明计划是否被执行，或表示“命运（即命中注定要发生的事）”，而非计划；was / were to have done sth.表示未曾实现的计划。例如：
I felt nervous because I was soon to leave home for the first time.我感到很紧张，因为我很快就要首次离开家了。
They said goodbye, little knowing that they were never to meet again.他们告了别，不知道以后再也不会见面了。
We were to have told you, but you were not in.我们本来想告诉你的，但是你不在家。
2.表示“应该”，相当于should, ought to。例如：
You are to report to the police.你应该报警。
What is to be done?应该怎么办呢？
3.表示“必须”，相当于must, have to。例如：
The letter is to be handed to him in person.这封信必须亲手交给他。
You are to do your homework before you watch TV.看电视之前你得先做完作业。
4.表示“想，打算”，相当于intend, want。例如：
If we are to be there before ten, we’ll have to go now.如果我们要在十点前到，我们现在就得走。
5.用于第一人称疑问句，表示征求对方意见。例如：
Am I to go on with the work?要我继续完成这项工作吗？
What are we to do next?我们下一步该怎么办？
6.用于否定句，表示“禁止”，相当于mustn’t。例如：
The books in this room are not to be taken outside.这个室内的书籍不得带出室外。
You are not to smoke in the reading-room.你不可以在阅览室里吸烟。
7.表示“可以，可能”，相当于may, can。例如：
The news is to be found in the evening paper.这条消息可以在晚报上见到。
Such people are to be found everywhere.这种人到处都有。
She is nowhere to be found.在哪里也找不着她。