a problem
He has not yet returned. He is not back yet. 第二句用be＋副词来代替完成时，why？ He has not been back yet.是否可以？
He has not yet returned。 He is not back yet。 第二句用be＋副词来代替完成时，why？ He has not been back yet。是否可以？ 个人认为第一句中yet是up to a specified time； thus far；up to now“迄今：直至一特定时间；至今”之意，这个意思通常用在完成时态当中，一般不用于一般过去时，再例： The end had not yet come。
He hasn't started yet。( not He didn't start yet。 ) 第2句中的yet是 at this time; for the present“现在；目前”之意。再例： She isn't ready yet。
虽然这两句中的yet在表达的时间上是不相同的；但现在完成时强调的是过去的动作对现在的某种影响或结果，所以两句话所体现的意思却是相同的，换句说，He is not back yet。和He has not been back yet。表达的是同一个意思。
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