English cases

>>Genitive is optional:

a friend of Joshua or a friend of Joshua's
driver license or driver's license
a friend of me and you or a friend of mine and thine<<

Not always, you can't for example say 'Joshua book' or 'The book of Joshua', it has to be Joshua's book. Although I suppose you could get round it with 'The book belonging to Joshua'.

Genitive case in English nowadays is used only when making a noun into a modifier to show possession.

There may be ways to get around it, but why go through all that round-about-hassle? Ence neet (=just use) the Genitive. It's eather (easier)

>>Genitive is optional:

a friend of Joshua or a friend of Joshua's
driver license or driver's license
a friend of me and you or a friend of mine and thine<<

Depends on the dialect; at least here, "a friend of mine" is obligatory, with "a friend of me" being ungrammatical. Similarly, the normal usage here is "a friend of Joshua's"; while "a friend of Joshua" is still grammatical, it does not really sound like what someone here would actually normally say, and it also implies a certain sort of distance between Joshua and his friend (due to the lack of the possession on Joshua's part).

I myself would probably take a middle position on this whole matter. I would say that there *is* a true genitive case in English, *but* it is not marked morphologically except on personal pronouns but rather is marked with the clitic "'s". The reason for this is that while from a syntactic point of view, "'s" has to be treated as a clitic due to its applying to entire noun phrases rather than to individual words, the way that it is used in phrases like "a friend of Joshua's" seems to imply that at some level it is still a case even though it is marked for entire noun phrases at a time (as in such cases one cannot treat "'s" as a adposition, and the way that it is used implies that there is still some limited case government by prepositions going on).