Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things (and are always capitalized).
Examples:
* Common Noun: native (refers to someone born in a particular place or belonging to a specific culture)
* Proper Noun: Native American (refers to a specific group of people)
* Common Noun: plant (refers to a general category of living organisms)
* Proper Noun: Rose (refers to a specific type of plant)
Let me know if you have any other grammar questions!