in

1.月或年的前面的in有时可以省去。例如：The book was first published (in) March 1951. 和The book was first published (in) 1951.

2.in a week, in three years等表达方式里的in有时作“在…以后”解，有时作“在…以内”解，比较下面每组里的两句：

• I shall see him again in a week(一星期以后).
  I shall see him several times in a week(一星期以内).
• He will graduate in three years(二年以后).
  He will learn another foreign language in three years(三年以内)

再比较下面两句：

• He will get married in a year（一年以后）.
• He will get married within a year（一年以内）.

see him again和graduate和get married都是一时的行为，see him several times和learn another language都是持续的行为。

3.in a week from now不及a week from now普通。in three years after his graduation不及three years after his graduation普通。

4.下面各种表达方式里都用in，不用on：

in the morning, in the afternoon, in the evening, in the night

但注意下面各种表达方式里都用on，不用in：

• on the next morning, on the following afternoon,
• on the preceding evening, on the night before,
• on the morning of 6 January, on the afternoon of his birthday,
• on the evening of her marriage, on the night of the event

5.the best novel that has appeared in years（几年来出版的长篇小说中的最好一本）和the coldest day in ten years（十年来最冷的一天）等表达方式是美国英语，在英国通常用for。

6.关于指地方的词前面该用in或at。参见at 1