1. at
1）用于表示在时间的某一点或某一时刻.例如:
What were you doing at this time yesterday?
Students often start classes at eight o'clock in the morning.
2）用于表示在三餐或中午、晚上.例如:
What did your mother say at breakfast ／lunch ／supper time?
We heard a girl singing in the next room at noon ／night.
3）用于表示节气、年龄.例如:
Children often receive presents at Spring Festival.
Jim began to play the piano at the age of seven.
2. in
1）用于表示在一段时间内.例如:
Mother will be back in a few days.
2）用于表示在非特指的早晨、上／下午、夜晚.例如:
My English teacher often gets up before 6o'clock in the morning.
My grandparents like watching TV in the evening.
3）用于表示月份、季节、年、世纪等.例如:
I was born in February, 1963.
Everything begins to grow in spring.
The Olympic Games will be held in Beijing in 2008.
The story happened in the 20th century.
3. on
1）用于表示在具体某天或某一特定日子的上午、下午或晚上.例如:
His mother was born on January 18, 1956.
She often does her homework on Saturday afternoon.
The old man died in the mountain on a cold winter afternoon.
2）用于表示节日.例如:
Children often go to church on Christmas Day.
People visit their relatives and friends on New Year's Day.
但是在含有early morning, late afternoon之类的词语时, 通常用in, 而不用on.例如:
in the early morning of 21st, March;
in the late afternoon of 15th, September
另外, 由last, next, this, that, these, those等词构成的表示时间的短语前面, 通常不用任何介词.yesterday, today, tomorrow, tonight等词前面也通常不用介词.例如:
He bought a Chinese-English dictionary this morning.
They traveled to Beijing last summer.
You'd better finish your homework tonight.