A. Noun clauses perform the same functions in sentences that nouns do:

A noun clause can be a subject of a verb:

What Billy did shocked his friends.

A noun clause can be an object of a verb:

Billy�s friends didn�t know that he couldn�t swim.

A noun clause can be a subject complement:

Billy�s mistake was that he refused to take lessons.

A noun clause can be an object of a preposition:

Mary is not responsible for what Billy did.

A noun clause (but not a noun) can be an adjective complement:

Everybody is sad that Billy drowned.

I hope this helps