英语that从句如果是被动句,动词be可以省略吗?

不可省略
解析如下:
被动语态中各种时态都需要"be"动词的变化来体现如下:
一般现在时：am／is／are＋done
一般过去时：was／were＋done
一般将来时：will／shall be＋done
过去将来时: would be+done
现在进行时：am／is／are being＋done
过去进行时：have／has been＋done
现在完成时：have／has been＋done
过去完成时: had been+done
从上可以看出被动句中的be是不可省的.要省除非你把主语和be动词都省略.
用非谓语形式的过去分词表被动.如
It is the book that was borrowed yesterday.
可转化成
It is the book borrowed yesterday .
其实如果没有上下文语境,提问者所例句子只能是一个强调句构:
即 "It is + 强调部分 + that ."
It is the book that was borrowed yesterday.
如果你把"was"省略掉.那么强调句还原成陈述句如下:
The book borrowed yesterday.
从上面可以看出,此句省略了"was"就相当于你把一个动词的承受者,变成了发出者,既不合逻辑又违背了原句句意.
If you still feel confused ,please leave message!

it is the book that I borrowed yesterday. 这样是对的，也可以去掉that，是先行词作宾语。你给的句子不正确。

这个应该不是这么说吧，borrowed 应该是过去分词作后置定语吧

不能，你举的这个例子是个强调句。

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