En skog = A forest
Skogen = The forest
Skogar = Forests
Skogarna = The forests
In other words, putting en/et(t) after a word is the same as putting a "the" in front of a word in the English language.
Den/det means "it" literally, but it never means "the". You can for example never say "Den skog brinner.", you should just say "Skogen brinner." ("The forest is on fire.") You can refer to the forest by replacing it with den/det though, as it means "it". For example: "Jag ser en älg, den springer." ("I see a moose, it is running.")
EDIT: I am bullshitting you. Den/det can indeed mean "the" when put it in front of an adjective. For example: "Den snabba katten" ("The fast cat") or "Den svenska tråden" ("The Swedish thread")
The only time
(EDIT: excluding the above case) you put den/det in front of a noun is when you want to specify exactly which <noun of your choice> it is.
Den skogen = That forest
Det huset = That house
You may add a "där" after den/det if you wish, but it doesn't change the meaning in any way. "Den där skogen" and "det där huset" still just mean "that forest" and "that house" respectively. It is most commonly used when pointing at something or when you just want to buy some time figuring out the following words.
And no, it can't be made into a postfix.
Oh, and:
Denna = den här = this
Detta = det här = this
Dessa = de här = these
However, there are some difference between the words, not when it comes to their meanings, but when it comes to grammar. "Denna", "detta" and "dessa" are much older ways of saying "this" and "these" than "den här", "det här" and "de här", resulting in people generally following older grammatical rules when using them. If you use "den här", "det här" or "de här" you always have to use the "en/et/<adequate plural postfix>" postfixes. You have to say "den här skogen" and not "den här skog".
However, when you use denna/detta/dessa, this is not the case. You can say both "detta hus" and "detta huset", the former being the older (and the most common, atleast where I live) usage (and the one that sounds better). In the case of "dessa", you always use the old way. For example, you never say "dessa stenarna", you say "dessa stenar" ("these stones").
