That-clauses are dependent clauses introduced (usually) by the word that which function as nouns.
Polly knew that her truck was almost out of gas.
That the storm was coming worried us all.
I love the theory that meteors brought life to earth.
Elvis was surprised that no one brought pie.
(Here, the That-clause is acting as the object of an elided preposition – Elvis is surprised (by the fact) that no one brought pie.)
Appositives: Nominal or Noun clauses can serve as appositives, as can certain relative clauses. Through transformational grammar, simple nouns also can serve as appositives.
Did you hear his comment that the story was fake?
“That the story was fake” and “his comment” are the same, more or less, so we call “that the story was fake” a re-naming or an appositive for “his comment.”
This is easier to see when we use transformational grammar with relative clauses.
My cousin who is a rocket scientist went to Australia last summer.
who is a rocket
scientist went to Australia last summer.
My cousin, a rocket scientist, went to Australia last summer.
Now we can see that “My cousin” and “a rocket scientist” are the same thing. If we replace “My cousin” with “a rocket scientist” we get a sentence that means the same thing (more or less).
My cousin went to Australia last summer.
A rocket scientist went to Australia last summer.
That’s the key test for an appositive – if the appositive can replace the word it renames and have the sentence mean, more or less, the same thing, then you have an appositive.
Which of these are appositives?
I threw a biscuit to my dog Heywood.
Elvis, the owner of the truck, came looking for its keys.
I gave cookies to Elvis, his best friend, and Ivy.
Our class is in Holt Hall, the oldest building on campus.
Interrogative are also noun clauses. These begin with interrogative words (why, how, who, which, if, what) and serve as nouns.
Ivy asked where her dancing bear had gone.
(Object of the verb)
Polly is worried about whether she will get the job.
(Object of the preposition)
What I asked is why you bought another guitar.
(Subject of the sentence, Subject Complement)