"The tail of the dog is long."
Translation:Hundens svans är lång.
We just don't like the av construction very much for speaking about parts of things. If you really, really want to have the tail first, we'd say svansen på hunden, but that sounds a bit special.
One reason we don't use av like you use of is that we use it for some other things that might cause confusion. For instance av is very often used for 'by', in cases like En bok av X 'a book (written) by X'.
en bild av is ambiguous in Swedish, it could mean either 'a picture of' or 'a picture by'.
Maybe I missed out on something, but what is the possessive form of) svans"?