What do you mean you were never taught the rules of the subjunctive? I thought they were part of the obligatory curriculum in Lingua Italiana. Or maybe you are too young and they don't teach that anymore. No wonder a young wife would go with "se tu venivi" instead of "se tu fossi venuto". I mean it's acceptable but only in very informal or friendly situations. As far as I was taught (of course, Italian as a second language), that's not correct grammar. I don't hear older (educated) people speak like that, nor journalists on TV nor politicians in, say, 'Porta a Porta'.
You are taught some Italian grammar in elementary school, but not that much.
And you were taught standard Italian, of course, which is not used in most informal conversation. It is way too formal. Journalists and politicians find themselves in formal situations, and therefore they must speak accordingly (not everyone can though!). The Italian linguistic situation is a bit complicated, and in these years a "new standard" is emerging, which is how we talk nowadays (once again, except in formal contexts).
When you start to see how people actually speak, you realise that many of the things that were not "correct grammar" at school are on the contrary widely used and acceptable.
And, as for the educated people: firstly, one doesn't have to be old to be educated; secondly, being educated doesn't mean to speak always in a formal perfect way, but it means adapting to the context. One has to know how to use the different variants of a language, ranging from, say, "ehi bello come butta?" to "Buongiorno signore, come sta?".
Creía que era fácil................................Credevo che fosse facile..........................................In this case, do you also slip into the imperfetto in an informal situation?
In this case it would sound weirder to me, and I wouldn't use the imperfetto. But I honestly don't know why!