Read the following sentence and find out the subject nouns in them.
David knows him since childhood

The correct answer is: David

A subject noun is a noun that is the subject of the sentence. A subject is a noun that performs the action in that sentence. In contrast, an object noun is a noun that is the object in a sentence i.e., the object noun is the receiver of the action in a sentence. 'David knows him since childhood', here subject noun is 'David'.