* 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.
While "German" refers to a specific nationality, and "submarine" is a specific type of vessel, the phrase "German submarine" doesn't refer to a particular, named submarine. It refers to a category of submarines belonging to Germany.
Examples:
* Proper noun: The U-boat (referring to a specific German submarine)
* Common noun: A German submarine (referring to any submarine belonging to Germany)