一、用in的场合
（1）表示“在某年/月/季节”这个含义时，须用介词in。例如： in 1980.
（2）表示“从现在起一段时间以后”时，须用介词in。 例如：They will go to see you in a week.
（3）表示“在某世纪”时，须用介词in. 例如:This machine was invented in the eighteenth century.
（4）表示” 在早上、下午、晚上“须用介词in. 例如: in the morning / evening / afternoon
二、用on的场合
（1）表示“在具体的某一天”或“（在具体的某一天的）早上、中午、晚上”等，须用介词on。
例如：on May 10th,1982，on a rainy morning， on a summer afternoon.
练习：
This festival is a cheerful occasion, for ____this day, the dead are said to return to their homes and they are welcomed by the living
这是个欢乐的日子，据说这一天，死者回家而活着的人欢迎他们
Mother's Day is ______the second Sunday of May．It’s a day to thank mothers． On that
day mothers usually get flowers and cards from their children．
（2）表示“在星期几”或“在星期几的早上、中午、晚上”等，须用介词on。
例如：on Saturday ， on Saturday morning, on weekdays在平时/周日
（3）表示“在某一节日”时，须用介词on。
例如：on Mid-autumn Festival, on Teachers' Day.
注意：当morning,evening,afternoon被of短语修饰，习惯上用in,而不用on.例如：
in the early morning of September 10th 在9月10的清晨；in the late afternoon of September 12th 在9月12日的傍晚。
三、用at的场合
（1）表示“某一具体时刻（即几点几分时）”，须用介词at。例如：at six
（2）用在特定的时候（时节、时机）时，须用介词at。例如：
They were happy at that time.他们那时很幸福。
（3）表示“在中午、在夜晚、在午夜、在周末（也可用on）”时，须用介词at。
例如： at midnight, at/on weekends
What do you often do at noon? 你中午经常做些什么？
You can see many stars in the sky at night. 夜晚你能看到天空中有许多星星。
（4）表示“在……岁”时，须用介词at。例如：
At the age of nine ,the boy could swim well. 在九岁的时候，这孩子就游泳游得很好了。
四、用by的场合
by+时间名词。意为：“到…时（已发生某事），此时谓语多用完成时；最晚、不迟于…，在…之前”。如：by now/then/this time/next Friday/the end of/three o`clock等。
By the end of last year,another new gymnasium had been completed.
到去年年底，又有一座新体育馆峻工了。
The train leaves at 6:00 pm.So I have to be at the station by 5:40 pm at the latest.
火车下午六点开，所以最晚我得五点四十赶到车站。
时间名词前不用介词的情况
1.在 today, tomorrow, yesterday, the day before yesterday, 等表示时间概念的名词前一般不用介词。例如:
1) What are you going to do tomorrow?
2) We have had six lessons today.
3) We had a good time the day before yesterday.
2.由this, that ,these, those 等用于表示时间概念的名词前,前面往往不加介词。例如:
1) That morning he got up very early.
2) What are you busy with these days?
3.由next, last, the next, the last 等用于表示时间概念的名词前时,前面不用介词修饰。例如：
1) We planted many trees last spring.
2) She said she would pay a trip to Suzhou the next month.
3) The very next time I saw her, she was working in London.
4.在each, any, every, some, all 等用于表示时间概念的名词前,一般不用介词来修饰。例如:
1) The buses go every 10 minutes.
2) Jack coughed all night.
3) We have to get up very early every/each day.
