Not rules, then, but "grammar conventions."
I know when I started learning Spanish, I was told there were certain grammar rules I couldn't break or I wouldn't be understood, and I've later found out that it wasn't true. Of course informal speech is very different than the written language. When you're first learning a language, though, you don't know what conventions within that language can be ignored and you might be hesitant to step out of those conventions.
And speech really does determine a language's conventions, I know - no one says "whom" anymore, and "Google" is a verb in the dictionary. :)
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