"No, it is not my juice."
Translation:Нет, это не мой сок.
Because "не" does not take the genitive case. If you have a sentence where "нет" is negating a noun, then the genitive case comes into play. But here we've got "не" negating "это" (basically meaning "it is", so it's effectively negating a verb), so the following noun stays in the nominative case.
I see that you asked this question a year ago and you're up to Level 20 in Russian, so you probably know the answer by now. But for the benefit of those who still don't, I hope this is of assistance.
«Нет, не мой сок» sounds weird to my ear (at least in this context).
These are different forms of "my". Possessives like мой, твой, ваш, наш and свой behave like adjectives and have different forms depending on the noun they modyfy. They take the gender (number) of the noun and follow its case. This is called agreement .
Мой (rhymes with "boy") is used with masculine nouns, моя with feminine, моё with neuter ones. Note that the plural мои is stressed on the last syllable.
3rd person possessives (его, её, их)!only have one form each.
Porque o significado é diferente:
- Нет, это не мой сок. = Não, isso não é o meu sumo.
- Нет, не это мой сок. = Não, o meu sumo não é isso.