Here's why:
* Nouns are words that name people, places, things, or ideas.
* Pronouns are words that stand in for nouns.
The simple subject is the main actor or topic of the sentence. It's what the sentence is about.
Let's look at some examples:
* The cat sat on the mat. (The noun "cat" is the simple subject.)
* She ran quickly. (The pronoun "she" is the simple subject.)
The other parts of speech you listed (verb, adjective, adverb, preposition) play different roles in a sentence:
* Verbs describe actions or states of being.
* Adjectives describe nouns.
* Adverbs describe verbs, adjectives, or other adverbs.
* Prepositions show relationships between words, often indicating location, direction, or time.