A simple subject can be either a noun or a pronoun. A simple subject consists of one word only, so either a noun or a pronoun could do it. For example: Mark is an excellent student. Here, the subject Mark is a simple one because it consists of only one word as opposed to in The tall boy is smart, where the tall boy is a compound subject, because it has more than one word. Or, if the subject is a pronoun, here is an example: He is very smart, where the personal pronoun he is a simple subject.