what does it mean to be stubborn anyway? I've heard the word but never ever used it.
According to Cambridge Dictionaries Online:
disapproving A stubborn person is determined to do what he or she wants and refuses to do anything else
Adjective has two functions:
Adjective as predicate (in a sentence). In this case, adjective doesn't end in "-e", e.g. Het meisje is koppig.
Adjective as a part of a phrase. Except for a neutral, singular, & indefinite noun, add "-e" for the adjective.
Zij is een KOPPIG meisje = She is a stubborn girl.
("Meisje" in this case is neutral, singular, and indefinite).
Ik zie het KOPPIGE meisje = I see the stubborn girl. ("Meisje" in this case is neutral and singular, but definite).
Hij is een KOPPIGE jungen. ("Jungen" is common --> de jungen).