Syntax of “There they all were, talking about homework and Quidditch and who knew what other rubbish”

Question

Hermione ripped open the newspaper and began to read the report inside while Harry looked around the Great Hall. He could not understand why his fellow students were not looking scared or at least discussing the terrible piece of news on the front page, but very few of them took the newspaper every day like Hermione. There they all were, talking about homework and Quidditch and who knew what other rubbish, when outside these walls ten more Death Eaters had swollen Voldemort's ranks.

_{Harry Potter and the Order of the Phoenix}

I'm wondering why it puts a comma between 'were' and 'talking'. I think "There they all were talking about homework" is fine. Another thing is about "who knew what other rubbish", which I feel it should be "who knew what other rubbish was". Any thoughts?

Answer 1

"talking about homework and Quidditch..." is acting as a descriptive gerund phrase. It is what they were doing while they were doing whatever the principal verb said. Here, the principle verb is were, meaning existing - but specifically existing there. That's it analytically, anyway.

What you actually have here is something of a set phrase. "There [noun/pronoun] [to be], [gerund phrase]" is the pattern. It is used to express or emphasise an emotional reaction to the fact people are doing whatever the gerund phrase says. This case involves Harry expressing surprise or disappointment that the world is becoming more dangerous, awful things are happening, but the other kids are mostly carrying on as normal.

"Who knows what other rubbish" is also something of a set phrase. There are variants, like "who knows what else", or just "who knows what", the real prototype of the construction. It is a stand-in when there is presumed or known to be something but the speaker doesn't know what it is - and possibly doesn't care.

Answer 2

There they all were, [talking about homework and Quidditch and who knew what other rubbish], when outside these walls ten more Death Eaters had swollen Voldemort's ranks.

The comma is used to separate "were" and the "talking" clause, since it's not intended that "were + talking" form the progressive aspect.

The bracketed element is a modifier in clause structure; more specifically it is gerund-participial clause functioning as a descriptive adjunct, giving descriptive information about the referent of "they". Note that it is interpreted with progressive aspectuality (as opposed to 'aspect'): "they were talking about homework and Quidditch ...".

Syntactically, the element "who knew what other rubbish" is a subordinate interrogative clause (embedded question), functioning as complement of "talking". But it's really just an idiomatic way of saying "it's anyone's guess what other rubbish".