Quindi dobbiamo sapere tutto questo?

if you want to speak and write proper german, you have to know these.

Well, yes. Initially. Once you become more fluent in a language you start forgetting all the grammatical rules as it just becomes second nature.

I’ve forgotten everything about English grammar I learnt in school. Native Germans don’t know much grammar either – a learner once asked me whether a phrase shouldn’t be in ‘Subjunktiv’, and I was like ‘Huh, what?!’. I don’t know why something has to be the way it is, I just know it has to 😛

But yes, initially you need to learn the rules.