"i" is beeing as conjunction between words like "and".
"a" is beeing used like very rarely in situation where you have to compare somerthing. Like in text "Europa a Polska" - which does not mean "Europe and Poland" but rather comparison between Europe and Poland/place of Poland (in Europe). If you want to say - pies i kot, you want to say about both of them, if you want to say "pies a kot" you mean to say that there is difference between dog and a cat and you want to compare them.
"a" is some kind of contradiction like "vs"