I disagree with the assertions below that "going down the street" or "goes down the street" would imply anything different in English than "walking down the street" -- the only difference is it's unspecified what mode of transport is involved, which is likely irrelevant or supplied by context.
Both "going" and "walking" are listed as meanings for идёт, but "Ivan is going down the street" isn't accepted. Is there an actual difference, or should it just accept either answer?
Doesn't идёт suggest that Ivan is going into a certain direction? I learned that ходить по улице means to walk through the street without any specific direction, as ходить doesn't specify direction and по is often used to indicate no specific direction. Does идти по улице have a slightly different meaning than ходить по улице?
I didn't really fully notice the curveball here, until revisiting this lesson after completing the whole course... OK, it should be second nature to me by now, but улице is in fact the DATIVE here, even though it happens to be identical to the PREPOSITIONAL form of УЛИЦА! Such sly geniuses at DL... And a reminder of the need to drill and review this stuff ad nauseum.
"Ivan goes down the street" in English kind of needs more information after it for the thought to be complete. It implies repetition and needs a qualifier afterwards. For example, "Ivan goes down the street to buy flowers" or "Ivan goes down the street to use the bathroom." So in English, qualifying phrase after. The repetition part is expressed in Russian with ходить. In this sentence may be better to stick with present participle, "is walking" as it's more exact and completes the sentence by not requiring further information after.
"Ivan goes down the street" works in English but it is just an incorrect translation on its own because it implies a complete trip e.g "Ivan goes down the street (every Monday)" [and surely goes back home after.] 'идти' is uni-directional and is only used talking about getting from A-B, unlike - as Dominic says - 'ходить' which is multi-directional and suggests a return trip: Today I went into town (home- town- home [A-B-A].