时间介词
时间介词，包含时间点和时间段两子类。
1. 表示时间点。常用介词on, in, at.
at:用于很短的时间和准确的数字前面。例如:
a. 用于具体的钟点前: at 10:00; at 5:45
b. 用于短暂的时刻前: at noon/ night/ midday/ midnight; at dusk（黄昏）/ sunset; at dawn（黎明）/ sunrise/ daybreak （黎明）
c. 用于进餐的时间前: at breakfast/lunch/supper（在早餐时/午餐时/晚餐时）
d. 用于年龄前面: at 14 =at the age of 14
e. 用于一些固定搭配中: at Christmas ·at New Year; at Thanksgiving（感恩节）
at first（起初） ·at last（终于）; at the moment此时此刻; at that/this time; at this time of last summer; at a good/bad time of year; at the beginning/end of this month;
on:用于某天或者某几天。例如:
a. 用于日期前: on May 20th; on September 10th; on the morning of October 1, 1997; on the first day of summer holiday; on a sunny morning.
b. 用于星期前: on Saturday; on Sundays/weekdays/weekends
on Monday morning; on the afternoon of Thursday
on weekends = on the weekend = at weekends
c. 用于节假日前: on Teachers’ Day; on Christmas Day／Eve; on National Day; on Tom’s birthday
