In Chinese you can say 我买了车 ("I bought car") without signaling whether the listener knows the car. English won't let you — before the noun you must declare: a car (any car / one the listener doesn't know yet) or the car (the one we both know). Articles carry the message "is this new or old information for you?"

One rule covers most of it: first mention takes a/an; after that, the; if it's already common knowledge, go straight to the. Everyone on Earth knows the sun — so it's always the sun.