* Nouns are words that name people, places, things, or ideas. For example: *cat*, *school*, *happiness*.
* Pronouns are words that replace nouns. For example: *he*, *she*, *it*, *they*.
* Determiners are words that come before nouns and modify them, indicating quantity, possession, or specificity. For example: *a*, *the*, *my*, *this*.
In the sentence "A cat is sitting on the mat," "a" is a determiner that specifies that we are talking about one cat.