While in the street, I fell down. in. It is not saying that he hit the street, it is saying where he was when he fell. "in the street" means that they are on the street itself, not inside, not in a garage, not in a shop, not in the mall, but actually in the area known as "the street". So, while in the street, I fell down.

Both are correct, but they mean slightly different things which is why prepositions are linguistic bottlenecks, especially in English. "I fell down in the street" in American English typically refers to the physical location where you fell, city streets are sometimes slightly below sidewalks as well "in the ditch, gutter," "I fell down on the street" is referring to the surface which you fell upon "on the grass, couch." There are other nuanced meanings for these statements, their accuracy depends on the meaning you are intending to convey.

摔倒在大街上，虽然on=在，，，上，但仔细体验一下中文意思，在大街，是指地方，像前面nanren888解释的那样，如果非要强调大街上，实际的意思是地上，马路上，英文里street和中文大街是有差别的，如果稍微改动一下， he fell down on the ground in the street,是不是容易理解了呢？ on 多指具体的东西上面，如桌子，地面，，，

The second one ;)

on I don t know explain why but I'm sure