И joins, unites words and clauses,
a contrasts them, singles them out, sets them aside.
Imagine a photo of Anna and Tom, a married couple. When showing the photo to your friend who doesn't know them, you can say:
Это — Анна и Том.
By saying this, you show them as a single entity, a married couple in our case. But you can say it with
Это — Анна, а это — Том.
Here you just name the two people in the photo one by one making no allusions to whether they are connected in a way or not,
a just separates them from each other, just the same way as if you said Это — Анна. Это — Том.
Now imagine that both of them like reggae music. You will tell your friend about it this way:
Анна любит регги, и Том тоже любит регги.
и because reggae is something that unites them. But in a similar situation, if there's something that sets them aside, you'll have to use
Анна любит кошек, а Том любит собак.
Анна всегда ездит на машине, а Том на велосипеде.
Or imagine that you and your friend are in a store, you point to different kinds of candies and explain to your friend which ones you like:
Эти конфеты мне очень нравятся, и эти нравятся, и те тоже, и те, а вон те я не люблю, они слишком кислые.
You like the first four kinds of candies, and that fact unites them, that's why you use
и when speaking about them, but then there's the kind you don't like, you set it aside from the previous ones by using
a. That's the main principles of how
И объединяет, а