"Pero" is used for a contrast when the first part of the sentence isn't negative. "Sino" is used for a contrast that starts negatively and is contradicted in the second half of the sentence. "Daniel has free time, but doesn't know what to do with it" In this instance, "but" is used to contradict the non-negative, which is where you would use "pero". "She wasn't late, but rather she was punctual." Translated into Spanish, you wouldn't use "pero" but you would be using "sino" instead, because the contradicted item is in the negative tense.