Why does «Он часто бегает по этой улице» translate to "He often runs ON this street", but «Две кошки бежали по улице» translates to "Two cats often run DOWN the street"?
You tell me. There is a very slight difference in English (I would probably use "on this street" to describe the habitual/usual occurrence but "down the street" to describe some movement or action happening now), but in Russian the context takes care of the difference.
To me, saying "down" or "on" have very little difference, and I could use them interchangeably, so I'm wondering if the course just doesn't recognize this, honestly. And that's exactly what I'm asking about, too: what exactly is the context that takes care of this difference in Russian? It's not clear from the exercises.
I think "running on the street", "running in the street" and "running down the street" should all be accepted.
There is a very slight difference with "running down the street" but not as you have described. It more gives the impression of running in a direction away from the observer, or passing by and continuing away from the observer. In fact it could be the least favorable translation in that I think the impression in Russian should be they were running around without any particular direction.
How would you say "Two cats ran outside?" That was my answer, but it was marked wrong.
"Две кошки убежали на улицу"
1. "бежать" is an imperfective verb. Your sentence would ask for a perfective verb such as as "убежать" - "to run off/away"
2. Outside (as an adverb) is translated to Russian as either "на улицу" (direction) or "на улице" (location). With any other preposition, e.g. "по улице", the word "улица" should be taken literally, i.e. it means "a/the street".
In American English at least, "street" and "road" can be used nearly interchangeably and should both be accepted.
"Down the street" is a fixed expression; no up/down/above/below is implied.