6 ideas
Dante holds that the vernacular a child picks up by imitating a nurse is more noble than 'grammatica' (Latin), because it came first, it is used by the whole world, and it is natural to people. Latin is a secondary, artificial language learned through rules and years of study, so it is stable but belongs to only a few.
Adam received from God a single perfect form of speech. It survived until the builders of the Tower of Babel were punished with confusion, and each work-group was given its own tongue, with the most skilled trades getting the crudest speech. Dante uses this to explain why human languages keep splitting and drifting over time and space, since after Babel no language is fixed by nature.
Dante names four qualities that the ideal Italian vernacular must have. It is illustrious because it shines and raises up those who use it. It is cardinal because every local dialect turns around it like a door on its hinge, courtly because it would be spoken in a royal court if Italy had one, and curial because it is weighed and judged like speech in a court of justice.
Dante surveys some fourteen regional dialects and finds that none of them is the illustrious vernacular, his own Florentine included. He compares the ideal to a panther whose scent is present everywhere but which lives in no single place. On this view a standard language is an abstraction drawn from the best usage of many regions, not the dialect of one city that has been promoted above the rest.
Dante says only poets of great talent and learning should use the illustrious vernacular. They should use it only for the three highest subjects: salus (prowess in arms), venus (love), and virtus (rectitude of will). Style has to match both the rank of the subject and the ability of the writer, the way the best horse belongs to the best rider.
Dante ranks the canzone as the supreme poetic form and sets out rules for writing one. The hendecasyllable is the noblest line, and the tragic style needs 'combed and shaggy' words, meaning ones that are polished yet sonorous, while childish, effeminate, and rustic terms are rejected.