助动词have的用法
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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