"They live in the city, but I live in the village."
Translation:Вони живуть в місті, а я живу в селі.
No, sorry, you can't use "i" here. I just said it with "i" couple of times and it sounds wrong and weird. It's not about comparison, "a" really means "and" and nothing more, not "but" or something. It is "and" for connecting statements only (not nouns out verbs or adjectives) that is used when the statements are different... Somehow, English has only I one "and" while we have two :) same as, say, Spanish has two verbs "to be" and for them they are utterly different and not interchangeable...