While reading a grammar book, I learned を may mark a.) a starting point of where a movement occurs, or b.) mark a location where a movement occurs. Examples:
a1.) 車｛くるま｝を降りました｛おりました｝。(He) got out of the car.
b1.) 危ない｛あぶない｝なあ！歩道{ほどう｝を走る｛はしる｝なよ！(That's) dangerous! Don't ride on the sidewalk!
However, other particles serve similar functions, and I don't quite understand the differences in usage. For example,
a2.) 車から降りました。
b2.) 危ないなあ！歩道で走るなよ！
Why would someone used を in the first sentence instead of から? Why would someone use を in the second sentence instead of で? How do the meanings in these individual sentences change between one choice over the other? When to use を in sentences like these and when to use different particles?
