The first example appears to be not flective at all, like in analytical languages. The second is sound more correctly to me, because it's using grammatical case... Am I right? The second is right and first is wrong? Thank you!
The first one is "buy the expensive cheese", the second one "buy some expensive cheese".
As its English counterpart, the first one can only be used in context, since "the" implies we know what cheese exactly are we talking about.
It might be used in a conversation like that:
— Купи сыру.
— Какой сыр покупать? Есть дорогой и дешёвый, но дешёвый невкусный.
— У нас сегодня гости. Купи дорогой сыр.
This distinction, though, only works for uncountable nouns.
The first example appears to be not flective at all, like in analytical languages.
It's used in accusative, which is an oblique case. It's more obvious if we use a 1st desclension noun with different forms in accusative and nominative:
Вода (nom.) / купи воду (acc.) / купи воды (gen.)
Technically we may use three cases here.
- Accusative Купи сыр
- Genitive I Купи сыра
- Genitive II (or Partitive) Купи сыру
Here the difference between Acc. and Gen. is referring to definite vs indefinite subject. That is купи сыр translates into buy cheese, while купи сыру into buy some cheese.
Choosing Gen. II over Gen. I is rather optional. Personally, I'd say "купи сыра" but it's probably the matter of dialect etc.