A noun clause acts like a noun. It can serve all the purposes a noun serves in a sentence. In the following examples we are comparing the five functions of a noun with that of a noun clause. The sentences are patterned on a similar fashion, but in the first one we have used a noun and in the second one we have used a noun clause.

As the subject of a verb

The news was true. (Here the subject of the sentence is the noun ‘the news’.)
What he said was true. (Here the subject of the sentence is the noun clause ‘what he said’.)

As the object of a finite verb, participle or infinitive

He told me a story. (Here the noun ‘a story’ is the object of the verb ‘told’.)
He told me that he would not go. (Here the noun clause ‘that he would not go’ is the object of the verb told.)

As the object of a preposition

Pay attention to my story. (Here the noun ‘my story’ is the object of the preposition to.)
Pay attention to what I say. (Here the noun clause ‘what I say’ is the object of the preposition to.)

Object of a verb of incomplete predication

My belief is true. (Here the noun true is the object of is which is a verb of incomplete predication.)
My belief is that he will pass. (Here the noun clause ‘that he will pass’ is the object of is.)

In apposition to a noun or pronoun

Tagore, the great poet, was also a great painter. (Here the noun ‘the great poet’ is used in apposition to the noun Tagore.)
The news that he was safe made us happy. (Here the noun clause ‘that he was safe’ is used in apposition to the noun ‘the news’.)