[析] On 加动名词表示"一……就"。本句的译文应是：我一进入教室就听见这个好消息了。又如：on hearing… 一听见， on arrival 一到达就……(on表示动作的名词)
[误] In the beginning of the book, there are some interesting stories.
[正] At the beginning of the book, there are some interesting stories.
[析] at the begining与at the end都是指某事物的开始与结束部分，均不指时间范围，而in the beginning 则是指开始一段时间。in the end=at last是指"最终，终于"之意。
[误] Till the end of next week. I will have finished this work.
[正] By the end of next week. I will have finished this work.
[析] by 引起的时间状语表示了动作的截止点，其意思为"不迟于某一时刻将工作做完"，所以主句一般是完成时态。当然可以有将来时态，如：I'll be there by five o'clock.而till则表达其一动作一直持续到某一时刻，但句中的动词一定要用持续性动词，而瞬间的截止性动词应用其否定句式，如：I won't finish this work till(until) next weekend.
[误] He came to London before last weekend.
[正] He had come to London before last weekend.
[正] He came to London two weeks ago.
[析] before 一般要与完成时连用，而ago则与一般过去时连用。
[误] I have studied English for three years gince I had come here.
[正] I have studied English for three years since I came here.
[析] since用来表达主句动作的开始时间，所以其引出的从句中应为过去时，而不能用完成时态
[误] I can help you repair this bike. You will get it after two hours.
[正] I can help you repair this bike. You will get it in two hours.
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