It's said that grave robbers had sacked Cao Cao's authentic tomb.

that引导的从句是宾语从句还是主语从句？从句时态受不受主句制约？我的理解是it是形式宾语，真正的宾语是that从句，因为是said的接受者，所以是宾语，请问是这样的吗？

【答】你的理解是对的！这是被动语态，原来主动语态的宾语变成了主语。

该句时态有问题。主句是一般现在时，从句根据需要可以不受限制。但是本句的had sacked不正确，应改为sacked或has sacked。

如果保留had sacked的话，那么把It is said改为It was said，也可以。

It's said that grave robbers sacked Cao Cao's authentic tomb. 

在实际语言中，一般不说：That grave robbers sacked Cao Cao's authentic tomb is said.

一般都说：People say that grave robbers sacked Cao Cao's authentic tomb.