"It was an admission that all might not be right between them." - that-clause as subject.. or not?

Question

“Here, snag my hand!” Alai called.

Ender held out his hand. Alai took the shock of impact and helped Ender make a fairly gentle landing against the wall.

“That’s good,” Ender said. “We ought to practice that kind of thing.”

“That’s what I thought, only everybody’s turning to butter out there,” Alai said. “What happens if we get out there together? We should be able to shove each other in opposite directions.”

“Yeah.”

“OK?”

It was an admission that all might not be right between them.

(Source: this ELL question)

What is the grammatical role of the that-clause here?

I first thought it's a noun clause serving as the subject of the sentence:

That all might not be right between them | was | an admission.

But such reading strips the sentence of its original meaning.

And is this really a noun clause?

Answer 1

Summary

The that-clause is a "content clause" that functions as a "complement clause" of the noun phrase by "postmodifying" it.

In the sentence,

It was an admission that all might not be right between them.

the part [ an admission that all might not be right between them ] is a "noun phrase". In this noun phrase, [ an admission ] is postmodified with the that-clause: [ that all might not be right between them ].

Discussion

This that-clause is not a relative clause. It is a content clause. To be more specific, it is a declarative content clause. According to Wikipedia, a content clause is a subordinate clause that provides content implied, or commented upon, by its main clause.

It is important to know the difference between the two constructions (relative clause and content clause). There are two important reasons.

First, they are different constructions. In the case of a relative clause, "one of its argument shares a referent with a main clause element on which the subordinate clause is grammatically dependent." (Wikipedia)

In the case of a content clause, its function is to complement, and it has no gap.

For instance, in "the person that saw you," the subject of the clause "saw you" is missing, but is understood to be "the person" that the NP as a whole refers to. Complement clauses do not usually have such a gap.
_{From What is a complement clause? (www.sil.org)}

Second, in order to be postmodified by a that-clause, the noun must license it. We cannot do this with any nouns. A noun that licenses a content clause as its complement is usually about a thought, an opinion, a belief, or information of some kind.

Here is the related part on the page that-clause by Cambridge Dictionaries Online:

Noun + that-clause
We use a noun + that-clause to express opinions and feelings, often about certainty and possibility. We also use that with reporting nouns. Some nouns commonly used in this way are belief, fact, hope, idea, possibility, suggestion, statement, claim, comment, argument:

He is also having intensive treatment in the hope that he will be able to train on Friday.
Dutch police are investigating the possibility that a bomb was planted on the jet.

Further reading:

• Content Clause - Wikipedia
• What is a complement clause?
• English Grammar: An Outline by Rodney Huddleston
• Nouns and Interrogative Complements, Linguistics Stack Exchange

Answer 2

The word that functions here as a subordinating conjunction: it introduces a subordinate clause saying what fact was admitted. As you inferred, the subordinate clause is not the subject of the sentence. The subject of the subordinate clause is all, and the subject of the sentence is it.

Here are some more examples of the same thing, with a verb before that:

Veronica admitted that she stole the cookies.

Jim said that he'd meet us at the movie theater.

The governor announced that she would run for re-election.

Karpov saw that there was no more hope of winning the chess match.

The data suggests that smoking causes lung cancer.

The contract requires that we pay within 30 days.

The word that can often be omitted from these sentences without much loss of clarity.

It works the same with a noun that stands for a proposition, like an admission:

The fact that water expands upon freezing causes icebergs to float.

The police chief released a statement that they have no evidence that the governor received a bribe.

Let's add a requirement to the contract that they must pay within 30 days.

Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.

Answer 3

I understand such sentences in terms of the verb that underlies the noun. I think of them as pseudo-passives, insofar as they have an implicit subject.

It was an admission that ... = It was an admission {by someone} that...

Someone was admitting that...

It was a concession that... = It was a concession {by someone} that...

Someone was conceding that ...

These nouns also have an implicit object.

It was an admission {by someone of something}

It was a concession {by someone of something}

It was an expression that caused him some consternation.

It was an expression {by someone of something} that caused him some consternation.