Why isn't it "DEN är en vägg."?
The general "it is" is always "det är" in Swedish. If you are talking about the "vägg" it will always be "den" though:
Jag har målat väggen. Nu är den grön*.
Are the pronounciations of ''väg'' and ''vägg'' the same ?
No, there is a long "ä" in väg and a short "ä" in vägg. You can watch this video by Blehg to learn about the differnt vowel sounds. And there are many in Swedish :).
If 'en vägg' is a wall then why is the Berlin Wall called 'Berlinmuren' på svenska?
Is it to do with the size of the structure? (I know mur is like the French, Dutch (muur) and similar to the German 'Mauer' as well.
I'm still having trouble with ett and en words