I've heard it said that Elizabeth is a tough businesswoman.

it在这里指的是：that Elizabeth is a tough businesswoman。
这里it是作形式宾语，真正的宾语是that Elizabeth is a tough businesswoman，由于它太长了，所以用it代替。
不可以换做是her，那是因为你没看懂结构。
hear sth done，听说某事被做。

这句话是个错句。I've heard 和it is said 重复了。
应为:
1、I've heard that Elizabeth is a tough businesswoman.
2、 it is said that Elizabeth is a tough businesswoman.
完全同意楼上的说法。

这句话是个错句。I've heard 和it is said 重复了。
应为:
1、I've heard that Elizabeth is a tough businesswoman.
2、 it is said that Elizabeth is a tough businesswoman.

I've heard it said that Elizabeth is a tough businesswoman.

没有错,我听它说,这个it指代说的媒体,可以是电视,广播等.

不能加is是因为语法,is只能领导三种词语,1.名词或形容词,2.动名词ing. 3.过去分词表示"被". 这里如果加了is就变成被说了.不符合上下文.

如果把it改成her完全符合语法,但是意思就变成听她说了,由非特指变成特指

it is said that 的it指代that后面的语句,
it said that 的it指代说的某媒体,而不是that后面的东西
因此I've heard it said that 意思是我听xx报道说了xxx

IT SAID听说的意思，不能改

IT IS