...as I knew he would.

[JungKim]

This is taken from an article:

I also mentioned this theory to my pal Eric Bickel from the Junkies the other day, and he immediately started nodding his head, as I knew he would.

Here, is it possible to add "that"?
...as I knew that he would.

 

[Keith Bradford]

Yes, but since "as" serves the same function it's rather superfluous.

 

[PaulQ]

[in the manner] which I knew he would nod [his head].

 

[JungKim]

Yes, but since "as" serves the same function it's rather superfluous.

If it's superfluous, why should it be still okay?

 

[Keith Bradford]

People often repeat themselves. It may be a bad habit but it's not a grammar fault.

 

[JulianStuart]

If it's superfluous, why should it be still okay?

Superfluous can mean excessive (with a connotation of "incorrect"), but it often just means unnecessary (but not "incorrect")

 

[JungKim]

What about this?
I also mentioned this theory to my pal Eric Bickel from the Junkies the other day, and he immediately started nodding his head, which I knew he would.

If this were the original sentence, would it be still possible to insert "that" after "knew"?
...which I knew that he would.

Is this "that" also superfluous but still grammatical?

 

[JulianStuart]

I don't think it's grammatical to include that that
It's a structure like
He did what he said he would.
He did what he said that he would.

 

[JungKim]

The relative pronoun 'what' might be a little different than other relative pronouns 'which' or 'that'.
So might you be saying that it's a structure like this?
He did something he said he would.
He did something he said that he would.