Once I've tried to explain but it's not easy. Here you can see something about it: http://www.italki.com/question/211232 Hope this will help you.

Tina did give a very useful answer at http://www.italki.com/question/211232 I have to admit that I never thought of "a" meaning "and" but now that I am aware of it, that is the closest English word that you can use. But this is not useful for learners at all. To build on what Tina wrote in her previous answer, I don't know if I'd describe it just as contrast. The true conjunction for contrast is "but", but I have to agree that there is a degree of contrast here. I think the key difference may be the lack of a direct connection that "i" provides. Let's look at her example of "Ivan radi, a Ana spava." (Ivan is working, and Ana is sleeping.). I'll just think out loud and let's see if something comes of it. - "Ivan radi a Ana spava" describes two unrelated actions by two unrelated people. I don't mean literally unrelated since this could easily be an answer to a question "What are your siblings doing?" My point is that their actions are unrelated and they are not one and the same person. One is doing one thing, the other is doing something else. If you had three of them, you would place "a" only before the last one. - "Ivan radi i Ana spava" doesn't make much sense because the two are completing different and unrelated actions. - "Ivan radi i Ana radi" is OK because it's the same action, even if not performed together. They are both at work and that "i" is connecting the fact that they are both working. - "Ivan radi i znoji se" works because these are two actions that he is doing at the same time. - "Ivan radi i Ana mu pomaže" also makes sense because he is working and she is helping him do that. - "Ivan radi a Ana mu pomaze" is fine as well, but somehow places a little more focus on Ana's action. (continued in the comments)

If you mean "a" and "i", this happens all across the Slavic languages. The basic difference is that "i" has the added meaning of "as well as" or "together with".