Corrective opposition (não... mas sim)

Setting the Record Straight

To correct a false assumption, use the structure não... mas sim ("not [this], but rather [that]"). It is the perfect tool for debunking myths and replacing them with facts.

The dinosaur was not a monster, but rather a normal animal.
O dinossauro não era um monstro, mas sim um animal normal.
That is not just a myth, but rather the truth.
Isso não é apenas um mito, mas sim a verdade.
This dinosaur is not a carnivore, but rather a herbivore.
Este dinossauro não é um carnívoro, mas sim um herbívoro.

Matching Prepositions and Actions

For the sentence to sound natural, the two things you contrast should balance. If the first half of the statement relies on a preposition (like a or de), you will typically repeat that preposition after mas sim.

That skeleton doesn't belong to a reptile, but rather to a bird.
Aquele esqueleto não pertence a um réptil, mas sim a uma ave.
The fossil is not of a lizard, but rather of the dinosaur.
O fóssil não é de um lagarto, mas sim do dinossauro.

You can also use this exact structure to contrast actions instead of just nouns or adjectives.

They didn't become extinct, but rather managed to evolve.
Eles não ficaram extintos, mas sim conseguiram evoluir.