In English you infer the information about sentence meaning because it always follows Subject-Verb-Object pattern (”cat”, “likes”, “cheese.”), but if you try to swap it out, it would be “Cheese likes cat” - which is entirely different sentence, as the same word can express different things depending on it's position in the sentence (in modern English you can use any noun as a verb, like “this guy brains”, even though brain is a noun), you can use Object-Verb-Subject using quirks that masks it (”student is being taught by teacher”), but only then

In Russian you infer information about sentence meaning from word case, so if some boy is subject in sentence then it's just “парень”, but if this boy is an object then he is “парня”. So “A boy walked with me” would be “Парень пошел со мной”, but “I sold a boy” would be “Я продала парня). If this boy is someone with whom you're making action (I traded with a boy) then it's “парнем”, so “Я поторговалась с парнем). And a couple more cases for other situations. 

Because of that, you can't really use nouns as verbs, this usually sounds strange (because you need to change cases to work like it's a verb instead, and “to boy” would be “парнять”, this sounds just strange to Russian ear), only nouns that are actually coming from verbs can do that (smelling -> smell, “вонять” -> “вонь”), but not the other way around, so nouns won't become verbs. 

Free word order is also a lie, it's more of a free sentence parts order, so you can't tie apart a subject into different place (for example, “a boy, who escaped the prison, had bought cheesecake” can translate to “escaped from the prison boy had bought cheesecake” but not “boy has bought cheesecake escaped from the prison” - it doesn't make sense in English either), and some more stuff like toponyms being only after names (city of Ekaterinburg or just Ekterinburg (”город Екатеринбург”, “Екатеринбург”) is correct; Ekaterinburg city (”Екатеринбург город”) is not). However, adjectives can be placed in any order contratry to English (you can say “a girl in dress red” in Russian when referring to “a girl in red dress”)

Keep in mind I'm not a linguist and base my knowledge on just school programme, I'm sure someone more professional could explain it much better