31.〔误〕He became a writter at his twenties.
〔正〕He became a writter in his twenties.
〔析〕这句话应译为：他在20多岁时就成了作家。在某人的一段生活时间段中要用介词in来表示，而在具体岁数时用at来表示。
32.〔 误〕We went to swim in the river in a very hot day.
〔正〕We went to swim in the river on a very hot day.
〔析〕具体某一天要用介词on, 又如：on New Years Day
33. 〔误〕Im looking forward to seeing you on Christmas.
〔正〕Im looking for ward to seeing you at Christmas.
〔析〕在节日的当天用on，而全部节日期间用at,Christmas是圣诞节期间，一般要有两周或更长的时间。
34. 误 I havent see you during the summer holidays.
正 I havent seen you since the beginning of the summer holidays.
〔析〕 during表示在某一段时间之内，所以一般不与完成时搭配，如：I visited a lot of museums during the holiday. 而for表示一段时间，可以用于完成时，如：I havent see you for a long time. 而through 用来表示时间时则为"整整，全部的时间"。如：It rained through the night.而since则是表达主句动作的起始时间，一般要与完成时连用。
35. 〔误〕At entering the classroom, I heard the good news.
〔正〕On entering the classroom, I heard the good news.
〔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)
36. 〔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。
37. 〔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：Ill be there by five oclock.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 wont finish this work till(until) next weekend.
38. 〔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。
39. 〔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。
40. 〔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. 〔析〕中文经常讲两小时之后来取，两天内会修好，而这个介词在英文中要用in而不要用after。其原因有二，①after 多用于过去时，如：I arrived in New York. After three days, I found a job in the bank. ② after 加时间是表达一个不确定的时间范围，如：after three days, 即三天之后的哪一天都可以。所以在许诺若干时间内会完成某事时，一定要用介词in。