"I am not a dog, I am a cat."
Translation:Nie jestem psem, jestem kotem.
I wouldn't say that. I started Dutch in Duolingo for fun in Jun, 2015. Finished the course in a few months which helped me with much of the basics so then I started watching dutch youtube videos and now (Nov., 2017) I can understand Dutch nearly as well English (my first language).
I don't get much practice speaking it so that needs work still, but that's me. I would say duolingo is great. And it's free, but still better than things I've paid more than $40 for.
"pies", "kot" - the basic, Nominative forms. Nominative is mostly used for the subject of the sentence.
"psem", "kotem" - the Instrumental forms. Instrumental is mostly used for "Y" in sentences built as "X is Y", when Y is a noun phrase. So basically "I am a doctor" or "A cat is an animal".
Cases. Those are two words with forms dependent on their function in the sentence.
"pies" and "kot" are Nominative - the basic forms, used mostly for the subject of the sentence.
"psa" and "kota" (I assume that's what you mean, "kotka" is the Nominative of 'female cat') are Accusative, used for the direct object. Accusative is needed for most of the verbs you learn in the beginning of the course.
"psem" and "kotem" are Instrumental. They are needed for example in sentences like "X is Y" (when both X and Y are noun phrases).
https://www.duolingo.com/comment/16296174 - you may take a look here and search for topics on cases.