Q:

Along/down the street

What is the difference between:
He was walking along the street,
and
He was walking down the street ?

A:

Actually both expression mean the same: a continuous movement on the road. In "along" is contained "long", so one may guess the idea is "(on) the length of the road" or in "along the river" "(on) the length beside the river". A verb of movement + down normally describes a mouvent from a higher point to a lower point. But it is also used for horizontal movement. 

