You are basically right, we have "iść + complement of place in instrumental" which means to go/walk somewhere. So we have e.g. iść ulicą, chodnikiem, drogą (which mean quite the same as iść po ulicy, po chodniku, po drodze (we use the locative case after "po" when it means on or over referring to places)). But back to iść + instrumental. We always use the instrumental here, nevertheless the sentence is negative because the word in instrumental is the complement, not the object of the verb. @DOWN: Thanks, no problem, anytime! ;-)
I'm now realizing that I will need more than Duolingo to even touch the surface of understanding Polish grammar. Not only is it difficult, but I have no idea what nominative, genetive etc means and I can't understand the explanations. Its like I am a preschooler reading college material. I learned most of spanish grammar by recognizing patterns (as well as living with spanish speakers). Its impossible to do that with Polish. Ugh! I want to give up, but I suppose I will just set a goal to just complete the course. That's reasonable, eh? Polish friends, is it common for even native speakers to write and speak incorrectly? LOL
"kawa" is Nominative (the basic form, used mostly for the subject of the sentence), "kawę" is Accusative (used for the direct object of most verbs), "kawy" is Genitive (used for the direct object of some verbs as well as when Accusative is negated).
"widzieć" (to see) takes Accusative. So "We see coffee" is "Widzimy kawę", but negated Accusative -> Genitive. "We do not see coffee" = "Nie widzimy kawy".