新东方考研语法讲堂142

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2.现在分词的被动语态(being +过去分词)通常表示“正在被…”的意思。如强调现在分词表示的动作发生在谓语动词所表示的动作之前时，可用现在分词被动语态的完成式(having + been +过去分词)。
     例如:
    The bridge being built will be completed next month.正在修建的那座桥将于下月完成。
    Having been kept out of the room about half an hour for his returning late，Tom was let in.汤姆回来晚了，所以被关在门外半小时左右才让他进去。