"The road is narrow."
~이 (~가 for nouns that end with a vowel) is the subject marking particle, and you use it when the subject of your sentence is someting in particular (길이 = the road, that specific road, none other than the road...), meanwhile ~은 (~는) marks the theme of the sentence (길은 = for what concerns the road, talking about the road...). The theme doesn't always correspond with the subject, but I guess that the difference between the two particles is evident in specific types of sentences and in daily dialogues, since they give different shades of meaning to the words they're referred to and to what you're trying to say. Hope this helps!
About the 조와요, I thought every descriptive verb ending with ㅂ changed it to 우. If were irregular, why that rule wouldn’t apply to it.
The ㅜ is just a weakened form of the ㅂ; it doesn’t “count” as a stem vowel. This means that the vowel in the ending depends not on the ㅜ but on the original last vowel of the unaltered stem. For example, the 해요체 form of the verb 돕다 “to help” (which is indeed irregular) is 도와요. The stem vowel is ㅗ, which means the vowel in the ending is ㅏ. The change from ㅂ to ㅜ (and the contraction with the following ㅏ) only occurs during the final linking stage, when the ending vowel is already determined. (EDIT: This part is incorrect, see below.)
Also note that 돕다 is a normal dynamic verb, not a descriptive/stative one, and yet it’s irregular. And there are others as well, e.g. 눕다→누워요 “to lie down” and 줍다→주워요 “to pick up”. But in general you’re right, generally speaking most descriptive/stative ㅂ-verbs are “irregular” and most dynamic ones are not.
As a side note, I don’t much like the “irregular” term here because it’s actually the norm for verbs ending in ㅂ to be “irregular” (yes, dynamic verbs usually don’t but there aren’t that many dynamic ㅂ-verbs – although some of them are very common, e.g. 입다→입어요 “to wear“ and 잡다→잡아요 “to catch”). So I feel it’s much more efficient to remember verbs like 좁다, 입다 and 잡다 which do not follow the rule which I regard as the normal one.
EDIT: After thinking about it a little more it seems I misremembered. It seems 돕다 as well as 곱다 (a rare word for “beautiful”) are the only ㅂ-“irrelooked it up again, and itgular” verbs that have ㅏ added. All others have ㅓ, regardless of the vowel before the ㅂ, for example 고맙다 “thankful” → 고마워; 새롭다 “new” → 새로워. I apologise for the confusion.