It is a grammatical structure made of a subordinating conjuction, a subject and a verb.
Nota:
It is also valuable to say that a noun clause is a dependent clause that it is subordinated to main clause. So it cannot stands by itself without annihilating the meaning of the sentence. If it were place alone, it would form a fragment, not a sentence.
It can be introduceb by the next subordinators:
That
Nota:
This subordinating conjunction is particular because is the only one that can be omitted when appers after a verb, otherwise, it cannot ommited. That is to say that, if it is in a N.C. working as a subject, it cannot be eleted: E.g.: "Tom doesn't know (that) you are married now". "That you were married shocked him".
WH-subordinators
Nota:
They are: "what", "why", "when", "where", "wether/if", "how". In contrast to that, they cannot be ommitted without lossing the meaning of the sentence.
Some examples: Tom wanted to know "how" to bake a cake.
Hillary asked me "wether" I wanted a cup of tea. "What" he's doing is none of your business. "Why" she did so is still a mystery.
What is it?
How can it be used?
Nota:
A noun clause is similar to a noun single word, so it behaves in the same way, and carries out the functions of: Subject, Object, Complement, etc.
Subject of a sentence.
Nota:
For instance: "What he said is not correct". We can see that "what he said..." is the subject, asking to the verb, "what is not correct?".
Object of a sentence
Nota:
It works as the object of a sentence when there is a transitive verb, i.e., one that need an objetc to complete its meaning. For instance: I don't belive "what she has told me". I don't swallow that she has cheated on that exam". Asking to the subject, "what don't you believe/ swallow?", we can see that the Noun Clause fits as the object of the sentence.
Object of a Preposition
Nota:
Sometimes, it's possible to have a noun clause as the object of a preposition. For instance: "I'm laughing at what he said". "I'm worried about what to say in the speech".
Apposition to a Noun
Nota:
An apposition is generally a construction that attempts to explain or state the same thing mentioned earlier in a different way. N.C. can be used in this way. They don't describe the subject, they are the subject expressed in other ways. So, be ware: "The fact <that the economy is getting worse> cannot be denied". That clause is not defining the fact, it is the fact. So don't confuse it. Usually, noun phrases in apposition are used with the following words: fact, belief, idea, doubt. Bear those words in mind.
Subject Complement
Nota:
A noun claus is usually used as the object of a linking verb. For instance: "It seems that he has never kissed a girl before". I'm what I everybody deslike, a witty tongue-in-cheek guy".
Adjective Complement
Nota:
As an adjective complement, the noun clause completes the meaning started by the adjective. For instance: "I'm sure that she's not happy". "The girl is sad that she had an argument with her best friend".
When is it used?
A noun clause is expected to be found when we report our thoughts or someone's words
Nota:
So, in reported speech (questions, suggestions, requests) we can expect to deal with them. For instance: "She asked me <where> I had bought that bag". "I thought <that it was a nice way to say goodbye>". "He suggested <that we should go for a walk>".
Sources
Nota:
1) "Communicate what you mean", C. W. Pollock, Prentice Hall Regents USA (1982). 2) Oxford Advanced Learners Dictionary.