"I" is used when you want to tell about two things or more. For example: I speak Polish and English. (Mówię po polsku i angielsku.) I like you and him. ( Lubię Cię i jego.) "A" is used as 'or'. For example. It's time to bed and you're still playing computer games. (Pora do łóżka, a Ty wciąż grasz w gry komputerowe.)

i - 'and' as a conjunctive (?), two or more things you are talking about have the same atribute, feature etc. a - 'and' as a way to change subject, differentiate, http://www.imaginarywars.com/wp-content/uploads/2013/09/And-now.jpg Pl: A teraz coś z zupełnie innej beczki - literaly and now something from completly different barrel bottom line - i together, a different.

"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"