Lundgren8 on Duolingo wrote:
The difference between ned and ner is that ned is used in writing and formal language and ner is the usual spoken variant.
However, both are equally accepted in writing. In certain compounds, ned can be common even in speech and you can say/write either nedladdning/nerladdning (downloading).For a few words, only ned- is possible in compounds such as nedlåtande ’condescending’.
PS : They are both derived from the older word neder (cf. Eng. ’nether’) which is used in a few words like Nederländerna (The Netherlands) and nederbörd (precipitation)
/ fluent 
/ learning 
/ fan of 
(bs/bl)