"Not fifty-two cities, but fifty-seven!"
Translation:Nem ötvenkét város, hanem ötvenhét!
For example, when you want to make sure you are not misunderstood in in a situation like the above.
With "két", you always need a noun that it refers to. "Két" cannot stand alone. "Kettő" can. "Két" also likes to be the first part of compound words and other derivatives. "Kettő" prefers to stand on its own.