Language / When do I use certain characters? [33]

Hello, I've searched everything and everywhere I can think of to find my answer. I'm using Duolingo to learn Polish. Duolingo does not teach you grammar at all, at least not with Polish. So I keep getting things wrong and I don't know how to correct them. I'm not sure when to use ę or y or a or whatever the case may be. For example: kobieta vs kobietę. Or mała vs mały vs małe. Any help or perhaps a link to a lesson would be great. Thanks!