"The young man lives in Rome."
Translation:Iuvenis Romae habitat.
Firstly, the word is 'iuvenis', not 'luvenis'. In answer to your question: iuvenis is the nominative and genitive singular; iuvenes is the nominative and accusative plural. I think iuvenis (pronounced with a long i at the end) can also be accusative plural, but I'm not sure about that one.
How do you know what nominative, accusative plural or genitive singulars are? Can you link a website where I can learn.
Also I assume genitive is gender and nominative is to do with names?
I hate to admit it, but learning Latin has showed how weak my English is. I didn't even know what a noun was until I searched it myself....
I actually learnt most of my Latin grammar from "Latin for Beginners", by Benjamin D'Ooge, which has some useful inflection tables in the appendices. You should be able to download it from https://www.textkit.com/ or, if you don't fancy searching through the scanned pages of an ancient textbook, you could try the Cambridge Latin Course (https://www.clc.cambridgescp.com/books/book-i).
You may want to check the meanings of all the cases, since unfortunately neither of your assumptions was correct. Using the genitive case is essentially like putting the word "of" in front of a noun in English, and the nominative is used to mark the subject - the noun doing the action described by the verb. English pronouns kind of use these cases: "I" is nominative and "my" is the genitive of the same word ("me" is best labelled as the accusative); similarly, "he" is nominative while "his" is genitive (and "him" is accusative"). D'Ooge explains it better.
Word order does matter; just not in the same way as in English.
Generally, use SOV (subject - indirect object - direct object - verb). (Here, the locative ("Romae") goes in the middle.) When using a form of esse (to be), it can be better to put the verb between the subject and its complement.
Because the case endings identify the subject/object/etc, you can change the word order; just don't do it randomly. If you want to emphasise a particular word, you should move it to the front of the sentence* (or, for slightly less emphasis, to the very end of the sentence). Alternatively, if it is an adjective, simply put it in front of its noun.
* For this reason, I believe, the verb moves to the front in an imperative sentence (a command) or a polar (yes/no) question.
bas sake buxbeze sabeDev