11.10 比较过去时与现在完成时

　　 她已从巴黎回来了。
　 She returned yesterday.
　　 她是昨天回来了。
　 He has been in the League for three years.
　　(在团内的状态可延续)
　 He has been a League member for three years.
　　(是团员的状态可持续)
　 He joined the League three years ago.　
　　( 三年前入团，joined为短暂行为。)
　 I have finished my homework now.　
　
　 ---Will somebody go and get Dr. White?
　 ---He's already been sent for.　
　 句子中如有过去时的时间副词（如 yesterday, last, week, in 1960）时，不能使用现在完成时，要用过去时。
　 （错）Tom has written a letter to his parents last night.
　 （对）Tom wrote a letter to his parents last night.