я хочу сока
Why is the phrase "I want juice" translated as "я хочу сока" instead of "я хочу сок"?
I thought masculine nouns didn't change in this case. For example, I believe it is "я хочу хлеб" and not "я хочу хлебa".
You can say "я хочу сок". It is correct and means that you chose juice as an uncountable liquid from other liquids to drink.
If you say "я хочу сока" it’s correct too. It means that you want some of juice.
And you can say "я хочу хлеба" with the same meaning.
I want juice=Я хочу сока (Partitive/Genitive case)
I want the juice=Я хочу сок (Accusative case)
I want bread=Я хочу хлеба (Partitive/Genitive case)
I want the bread=Я хочу хлеб (Accusative case)
I thought masculine nouns didn't change in this case
Nowadays Partitive is almost universally replaced by Genitive case, but the good old Partitive was harsh with the masculine: Хотите ча́ю или со́ку? - Ча́ю, пожалуй. - Са́хару Вам положить? - Нет, спасибо.