AIDS is said to have been the biggest health challenge to bo
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首先,这不是被动语态,It is said to be .是固定结构,译为据说.这里的AIDS是形式主语,said 后面的不定式to 后面的整个句子为真正的主语.整个句子翻译出来就是:据说艾滋在过去的几年里已经成为男性与女性之间最大的健康挑战问题.

然后此句主句时态为一般现在时,从句有over the past few years便是现在完成时,所以为to have been.

诸如此类的还有

It’s supposed to be very good. 据说它质量很好.

It’s expected that the war would end soon. 预计战争不久即可结束.

It is reported that another earth satellite has been put into orbit. 据报道又有一颗卫星上天了.

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