If you say 'walk down the street', it means 'walk ALONG the street'. 'Walk down' doesn't mean go down, in the sense of descend. It just means to 'walk somewhere.' It doesn't have anything to do with being on a hill, although I can see how this might be confusing. For example My friend and I were walking down the street My friend and I were walking along the street. The meaning is the same. 'Walk on the street' is more specific

Sorry I submitted too soon. 'Walk on the street' is a more specific term, but it is less frequently used. It means to specifically walk on the pavement (or sidewalk, which is the US term.) e.g. Don't walk on the road, walk on the street instead. A bit more about 'walk down'. Its a peculiarity of the English language, phrasal verbs are often like that where you can't work out the meaning from the two words alone. For example a person saying 'walk down that road', means walk along the road, or go along that road, it doesn't mean to descend.

They're almost the same, but not quite. The phrase "You can walk on the street" is hardly ever used, and would mean that you can walk right on the street. Usually, we'd say something like "you can walk down the street". It is probably confusing because the person walking is not really going up or down. You can also say "you can walk up the street", or "I walked down to the corner" or "let's walk down to the store." Hope this helps!