31. 〔误〕He became a writer at his twenties.
〔正〕He became a writer 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〕I’m looking forward to seeing you on Christmas.
〔正〕I’m 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 haven’t see you during the summer holidays.
正 I haven’t 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 haven’t 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)