功能：
（1）表示过去某个特定时间或某一段时间发生的动作或情况。
We visited the school last spring.
I went to school by bike when I was in middle school.
China was founded in 1949.
（2）在表示时间或条件等的状语从句中代替过去将来时。
She told me that she would not go with us if it rained the next day.
They would not leave until she came back.
My friend promised to marry me once she made her final decision.
一般过去时，现在时和过去时的几组差异：（别以为这很简单，下面的差异你不一定明白。）
Her brother was a chemist.（已去世）
Her brother is a chemist.（尚健在）
That's all I had to say.（话已说完）
That's all I have to say.（言之未尽）
It was so nice to see you.（离别时用）
It is so nice to see you.（见面时用）
Jane did a lot of work this morning.（已是当天下午或晚上）
Jane has done a lot of work this morning.（仍是上午）［本句现在完成时，此乃后话！］
