Here's why:
* Nouns are words that name people, places, things, or ideas (e.g., "dog," "city," "table," "freedom").
* Pronouns are words that replace nouns to avoid repetition (e.g., "he," "she," "it," "they," "we").
Examples:
* The cat sat on the mat. (The subject "cat" is a noun)
* She went to the store. (The subject "she" is a pronoun)
While most subjects are nouns or pronouns, there are some instances where other parts of speech can function as subjects.
For example, gerunds (verb forms ending in "-ing" that act as nouns) can be subjects:
* Swimming is good exercise.
However, these are exceptions, and the most common and basic form of a subject is a noun or pronoun.