"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!
이 is the subject particle and 은 is the topic particle. They both mean the same thing though.