1．at表示片刻的时间，如：at 8 o'clock，常用词组有：at noon, at night, at midnight, at the end of, at that time, at the beginning of, at the age of, at Christmas, at New Year等。
2．in表示一段的时间，如：in the morning, in the afternoon, in the evening, in October, in 1998, in summer, in the past, in the future等。
3．on总是跟日子有关，on Monday, on Christmas morning, on the following, on May Day, on a warm morning等。
