Good or Good?
Jeg forstår ikke når man bruker 'bra' eller 'godt' i samtalen. Jeg hører 'det er en god ide' og 'jeg har det bra.' Is there a grammar rule for when to use specifically bra or god/godt?
In the distant past "bra" was only an adverb (meaning well) and and "god" was only a adjective (meaning good). Now each of the two words is used as both an adverb and an adjective so the distinction you imagine does not exist in normal speech. Although the following sentences should not be perceived as having identical meanings, to a great many Norwegians, they do.
Maten smakte godt! / Maten smakte bra!
Han er en bra person! / Han er en god person!
Det var godt du kom! / Det var bra du kom!"
I don't think there is such a rule, it's more about custom. Most of the time you can use either. Some immigrants use 'bra' all over and it works fine although it sounds slightly off at times (god ide sounds better than bra ide).
If someone is looking for a rule, it would be safe to say that "bra" and "god" are now both accepted as adjectives and adverbs; however, for native speakers, as a consequence of the words previously being only an adjective (god) or only an adverb (bra) in some circumstances their previous use has become so customary that if the other word is substituted its use seems strange.
Language changes. My 1950s teachers would be aghast to know that this generation of students is not taught that "god" is an adjective, "bra" is an adverb and that they cannot be interchanged. I'm glad your girlfriend reacts as she does. I wonder if she will have a positive impact. There are many times when I hear "bra" being used as an adjective that my internal language monitor sounds an alarm. But, unlike your girlfriend's reaction mine is usually internal.
I received an email in which astrokid124 evidences wonderment in respect to the uses of "god" and "godt". But, I can't find the message here. Regardless, I'll provide some information. "God" is used with masculine and feminine gender words. "Godt" is used with neuter gender words.