英语难点语法详解：助动词have
1)have +过去分词，构成完成时态。例如：
He has left for London. 他已去了伦敦。
By the end of last month， they had finished half of their work. 上月未为止，他们已经完成工作的一半。
2)have + been +现在分词，构成完成进行时。例如：
I have been studying English for ten years. 我一直在学英语，已达十年之久。
3)have +been +过去分词，构成完成式被动语态。例如：
English has been taught in China for many years. 中国教英语已经多年。
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