现在完成进行时态与现在进行时态的区别是什么?例如i have been writing a letter和i am writing a letter例如i have been writing a letter和i am writing a letter有什么区别
问题描述：
现在完成进行时态与现在进行时态的区别是什么?例如i have been writing a letter和i am writing a letter
例如i have been writing a letter和i am writing a letter有什么区别
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