1) 两个动作如按顺序发生，又不强调先后，或用then，and，but 等连词时，多用一般过去时。
When she saw the mouse，she screamed.
My aunt gave me a hat and I lost it.
2 ) 两个动作相继发生，可用一般过去时；如第一个动作需要若干时间完成，用过去完成时。
When I heard the news, I was very excited.
3) 叙述历史事实，可不用过去完成时，而只用一般过去时。
Our teacher told us that Columbus discovered America in 1492.
