I'm afraid you're confused, NedLong. There is no preposition in "No sé tú...". "No" is an adverb and "sé" is from the verb "saber". "No sé tú, pero yo no dejo de pensar..." = "No sé si tú dejas de pensar, pero yo no dejo de pensar...". Both "tú" and "yo" are subject pronouns (subject of the verb "dejar").Guessing from what has been said here that the title of Armando Manzanero's famous song would more correctly be 'No sé ti' instead of 'No sé tú'?