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现在完成时表示过去某时发生的行为对主语目前产生的影响。即用过去发生的某个行为来说明现在的某种情况。
We are good friends.(现在的情况）
I knew him in 1997.（过去的动作）
I saw this film yesterday. （强调看的动作发生过了。）
I have seen this film. （强调对现在的影响，电影的内容已经知道了。）
Why did you get up so early?（强调起床的动作已发生过了
Who hasn't handed in his paper? （强调有卷子，可能为不公平竞争。）
She has returned from Paris. 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.
