The output of the paper mill is now three times as high as it was in 1966.此句在in 1966前面用了it was.为什么呢?the irrigated area in this place is four times bigger than in 1978.为什么这句中in 1978前面没有it was 两句话加不
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The output of the paper mill is now three times as high as it was in 1966.此句在in 1966前面用了it was.为什么呢?the irrigated area in this place is four times bigger than in 1978.为什么这句中in 1978前面没有it was 两句话加不
The output of the paper mill is now three times as high as it was in 1966.
此句在in 1966前面用了it was.为什么呢?
the irrigated area in this place is four times bigger than in 1978.
为什么这句中in 1978前面没有it was
两句话加不加it was的区别在哪?
The output of the paper mill is now three times as high as it was in 1966.此句在in 1966前面用了it was.为什么呢?the irrigated area in this place is four times bigger than in 1978.为什么这句中in 1978前面没有it was 两句话加不
as it was in 1966 是一个比较状语从句.it 指the output.这里的it was 可以省略.
the irrigated area in this place is four times bigger than in 1978.这里than后面省略了it was（即省略了主语和系动词）.
省略不省略it was意义没有舒适区别,不省略显得更正规,省略则显得更口语化.又如：
She is as beautiful as （she was） five years ago. 她跟五年前一样美丽.
Whe is as beautiful as (she was ) in 2003. 她跟2003年时的样子一样美丽.
it was 应该是(that) it was，指的产量。第二句应该是than (that) in 1978。两个都是省略句。
第一句省掉的是what而不是that（相同的错误我昨天也犯过），如果要用that，后面的it was 就不能用了（that作代词）回指the output
第二句加上it was或what it was或者单独加that都可以
总而言之，两句都是一样的，加不加都可以
为谨慎起见，特申明是我个人的观点，仅供参考。...
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第一句省掉的是what而不是that（相同的错误我昨天也犯过），如果要用that，后面的it was 就不能用了（that作代词）回指the output
第二句加上it was或what it was或者单独加that都可以
总而言之，两句都是一样的，加不加都可以
为谨慎起见，特申明是我个人的观点，仅供参考。
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第二个as后是比较状语从句，比较状语从句的结构应和主语结构一致，it代替The output of the paper mill，因为后面有in 1966，所以在it后加了was。