A noun clause is a collection of words, one of which must be a noun, which plays the role of a noun. As with a noun, the noun clause can be a subject, an object, or a complement. Although some clauses are independent clauses – i.e they can form a complete sentence if on their own – the noun clause is always dependent clause. It cannot stand alone as a complete sentence.

Examples of the noun clause

“What I knew about the incident helped the police piece together the crime.”

Let us break down  the sentence.  Firstly, we know that the verb is ‘helped’.  There are two nouns contained within the noun clause which is the subject.  They are ‘I’ and ‘the incident’.  Neither of these work as the subject in the sentence structure, and if we study who or what helped the police, we can figure out how the subject can only work as a noun clause:
Why is the noun not ‘I’?
Did ‘I’ help the police?   No
So, why not ‘the incident’?
Did ‘the incident’ help the police?  No
Okay, let’s look at the noun clause:
Did ‘What I knew about the incident’ help the police?  Yes.

Note the sentence’s noun clause begins with the word ‘what’.  Words like this are clues that a noun clause is being used.  Other such words such as who, whom, whose, which, that, if, whether, what, when, where, how, why, whoever, whenever, whatever, wherever are also indicators of a this clause:
“What I knew about the incident helped the police piece together the crime.” Noun clause as subject.
“I drove whenever I could.” Noun clause as object.
“We are so excited to hear that it is snowing today” Noun clause as subject complement.

A noun clause is a collection of words, one of which must be a noun, which plays the role of a noun. As with a noun, the noun clause can be a subject, an object, or a complement. Although some clauses are independent clauses – i.e they can form a complete sentence if on their own, a noun clause is a dependent clause and it cannot stand alone as a complete sentence.