The subject of a sentence may be a noun or a pronoun.
Check these sites for information about sentence structure and parts of speech.
Ms. Sue is correct and she's given you some excellent websites.
Remember that almost all subjects are either nouns or pronouns, but once in a great while a gerund phrase or an infinitive phrase or a relative clause can serve as a subject. Those are rare, but they do happen.
Notice the 1st example.
Notice the 3rd and 4th examples.
Scroll down until you see the section on NOUN CLAUSES -- there are examples there.