Here's why:
* First person: "I" and "we" refer to the speaker.
* Second person: "You" refers to the person being spoken to.
* Third person: "He," "she," "it," and "they" refer to someone or something other than the speaker or the listener.
Indefinite pronouns, like "someone," "everyone," "nothing," and "all," don't fit neatly into any of these categories. They don't refer to a specific person or thing.
Example:
* First person: "I went to the store."
* Second person: "You should go to the store."
* Third person: "He went to the store."
* Indefinite pronoun: "Someone went to the store."
We don't know who "someone" is.