what is noun clause and when do we use it?

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A noun clause is an entire clause which takes the place of a noun in another clause or phrase. Like a noun, a noun clause acts as the subject or object of a verb or the object of a preposition, answering the questions "who(m)?" or "what?". Consider the following examples:

noun
I know Latin.
noun clause
I know that Latin is no longer spoken as a native language.
In the first example, the noun "Latin" acts as the direct object of the verb "know." In the second example, the entire clause "that Latin ..." is the direct object.

In fact, many noun clauses are indirect questions:

noun
Their destination is unknown.
noun clause
Where they are going is unknown.
The question "Where are they going?," with a slight change in word order, becomes a noun clause when used as part of a larger unit -- like the noun "destination," the clause is the subject of the verb "is."

Here are some more examples of noun clauses:

about what you bought at the mall
This noun clause is the object of the preposition "about," and answers the question "about what?"