why did I get it wrong, putting i instead of a?
і is used to connect similar things ("This is mom, and that is mom, too"). When juxtaposing things that are different, or doing different things, you use а.
Try thinking of "а" as being closer to "but" rather than "and"
The tips and notes on letters 1 now has it explained
So 'a' is used as a coordinating conjunction for clauses.