"Mom, I want a snake!"
Translation:Мама, я хочу змею!
Since the verb tells you who is doing the action the pronoun should not be required. At least that is my understanding from the three years of Russian I took more than forty years ago.
I put in "мама, я хочу змея", and it accepted it as a typo even though I used nominative instead of accusative. Should count as incorrect.
Slavic names have short forms, ‧ Short forms by themselves can form "reemerging" vocative case (sometimes called neo-vocative); it is used for calling a familiar person, substituting nominative singular by removing last vowel ‧ there is a neo-vocative form for close relatives: мать – ма́ма – мам ‧ en.wikipedia.org/wiki/Russian_declension ‧
Slavic vocative has been lost in Russian and is now used only in archaic expressions ‧ en.wikipedia.org/wiki/Vocative_case ‧
used to be seven cases in Russian, the seventh case being the vocative ‧ russianmentor.net/gram/mailbag/topics/nouns2.htm ‧
What part of speech is Mom in this sentence and why do we use nominative case here? I thought it would be accusative.
You are addressing your mother, so normally that would be a vocative. According to the dictionary, the Ukrainian vocative of мама is мамо. I wonder why they use a nominative instead.
It could either be vocative or nominative. The vocative of мама is мам. duolingo accepts this answer