Is they a proper noun or common noun?

"They" is a pronoun, not a noun. Pronouns are words that stand in for nouns.

Here's why:

* Pronouns replace nouns to avoid repetition: "The students are happy. They are excited about their trip."

* Proper nouns are specific names of people, places, or things (e.g., John, London, Eiffel Tower).

* Common nouns are general names for people, places, or things (e.g., student, city, tower).

Let me know if you have any other grammar questions!