Ex. Ich spreche gut deutsch.
He then asked what "deutsch" was in this sentence. I was under the impression that it was a noun despite the weird lack of capitalization, since it seems natural to me to think of the language you're speaking as the object of the verb "to speak", but he said it was an adverb. Apparently my understanding of "adverb" is severely lacking, because I'm just not getting how "deutsch" could ever be an adverb, let alone in this particular sentence. Is this actually a difference in the labeling of parts of speech between German and English, or something else?
Also, would it make any difference if the sentence had been "Ich spreche gut Deutsch" with an uppercase 'd'?
Similarly, about a week ago, my prof was explaining the difference between what he calls primary and secondary adjectival endings and wrote out something very similar to these two sentences:
Ex. 1: Mein Auto ist blau.
Ex. 2: Mein blaues Auto fährt schnell.
My understanding, which is based on my last two semesters of German with a different professor, is that the second example requires an adjective ending because the use is attributive, whereas "blau" is a predicate adjective and so doesn't require any dressing up.
Current prof says "blau" is used as an adverb in 1 and as an adjective in 2. Does he just mean something different when he says "adverb"? My ability to produce these sentences isn't really suffering for his explanation, but I am pretty confused whenever something I think is a noun or an adjective gets called an adverb during one of our in-class exercises.
Any help you could offer would be super! If I've made any stupid mistakes, I apologize. I'm not having a great grammar day.