"The water is flowing down the wall."
Translation:Vattnet rinner nedför väggen.
You can use either in writing, ned being a bit more formal. In speech you’ll mostly find ner. Ned- is also common even in speech in some compound words like nedräkning (count-down), but you’ll also hear nerräkning. Same goes for nedladdning/nerladdning (download). In these compounds usually ned- is preferred in writing however.
What difference, if any, is there between Vattnet rinner nedför väggen and Vattnet rinner ned väggen
Is ned a preposition that doesn't come directly before a noun? Like i vs in?
ned is an adverb. It can function as a verb particle when stressed.
nedför can be either a preposition or an adverb.
in is also an adverb + often functions as a verb particle.