Can I use the "s" in any name form? For example: pojkes, pojkens, pojkars and pojkarnas
Generally, yes, as long as the word doesn't already end in an s. :)
Does "a boy's sandwich" translate as "en pojkes smörgås"?
So the -s- ending can be used in every word to show the possession? for example: "Kattens apelsin", "Hundens ost" and "Kvinnas barn", "Mans jordgubbe"? What about the words that end in -s?
Yes, with only a few mostly archaic exceptions. If it already ends in an s, you don't need to do anything. If it ends in an s and it's really, really ambiguous, you can write s' instead - but it's strongly recommended that you rewrite the sentence to make it less ambiguous rather than that.
I wrote the boy his sandwich. Isnt that correct in english? I know it is not the common way to write it, but I thought it was still good. (as in where the " 's" derives from
No, it's completely ungrammatical.
It's not actually from "his" or similar. Old English used to have a -es suffix for the genitive singular, and the e in it was gradually replaced by an apostrophy under Norman influence in Middle English.
What you're referring to is called "his genitive" and is not used in English. You can read a little more about it here: https://en.wikipedia.org/wiki/His_genitive
"The boy's sandwich" is an abbreviation of "the boy his sandwich", yet the latter one is considered incorrect
English doesn't work like that. Putting "the boy his sandwich" is ungrammatical. And "the boy's" is not an abbreviation of "the boy his" - the possessive singular in English is derived from an old English genitive inflection.
"Pojkens smörgås" means the boy's sandwich. But what if I want to say "the boy's sandwiches". Does it become "Pojkens smörgåsar" or do I have to change "pojkens" aswell? Does "pojkens" change if I'm talking about "the boy's sandwich" or "the boy's sandwiches"?