"We walk down the street."
Translation:Vi går ned ad gaden.
If you walk down getting out of the street you'd say " vi går ned af gaden", otherwise if you walk on the street following the way through the street you say "vi går ned ad gaden". In another sentence with"ad" from this section a dane explain incredibly well when to use "af" and "ad".
The Danish word "ned" is an adverb, not a preposition, so you cannot put it immediately before a noun: you need a preposition between the adverb and the noun, in this case "ad".
The word "ned" does translate into English as "down", but "down" in English can be either an adverb (as in the sentence "I'm coming down") or a preposition (as in the sentence "We walk down the street"), whereas Danish "ned" can only be an adverb. Another thing to keep in mind is that "ned" means moving down, as in "Jeg kommer ned", and if you want to say "down" in the sense of a fixed location, then it is "nede": "Jeg sidder nede i kælderen" means "I'm sitting down in the basement." A few other adverbs follow the same pattern of motion vs. fixed location: op/oppe, ind/inde, ud/ude, hen/henne.