Translation:They eat sour cream, but this is not sour cream.
Easy for me but my mother language is Polish. We have very similar pronounciation to Ukrainian words, some of them are Russian though.
Try to distinguish that when you give the name you're saying in normal way /nominative case like 'сметана', but when you speak about it - usually after a verb - you need to change it a little in this case 'сметану' /accisative case/.
English, portuguese, spanish do not have this sense of nominative/accusative... Or... Almost!
We can see nominative/accusative in action through pronouns like I, HE, SHE, WE in phrases like:
"I see you in this forum" - here, the pronoun I is the subject, it is in the NOMINATIVE case.
"You see ME in this forum" - here, the pronun I takes the form ME because it is the object of the verb "SEE".
If I write "you see I in this forum", you will say the phrase is wrong! The same occurs in languages like ucrainian, russian, german, latin, greek if we use nominative where we should to use accusative! :)
Use nominative when you must answer to question " WHO is eating/doing/seeing/etc. ".
Use accusative when you must say question like " WHAT are you eating/doing/seeing/etc. "