If you speak ungrammatically, I won't understand you. If you become an English teacher, for the rest of your life people will apologize to you for their bad grammar. But almost all people feel more or less grammatically inadequate. This is even though in the technical sense they all speak with perfect grammar. Usage, as a marker of dialect and social prestige, is what's at stake, not the technical grammatical rules of the English language.
Usage is serious business.
Usage, however, is so fluid that there is often no way to study for a usage test except to study directly to that test. If the New York Times and Oxford University Press approve a split infinitive but the test you're taking doesn't, you are advised to do as the test does. My imposition of that preference on you, or my censure of you for adopting another preference, can stand for the whole cultural work of usage prescriptions.