Pero vs Sino
A conjunction connects two or more sentences, clauses, or parts of clauses. "Pero" and "sino" are coordinating conjunctions meaning "but." However, while both "pero" and "sino" are used to express contrasting ideas, they have a slightly different meaning and usage.
PERO (but): it’s usually used when the second clause adds a qualification of what has been said before. It could also be translated as 'however' or 'nevertheless',
No soy inglés, pero hablo bien el idioma. I'm not English, but I speak the language well.
Es un día soleado pero hace frío. It's a sunny day but cold.
Mi primo es muy listo pero no es agradable. My cousin is very clever but not friendly.
SINO (but, but rather, but on the contrary,but instead): it’s used to say something negative or the opposite or contradicts something said earlier.
No soy alto sino bajo. I’m not tall but short.
Pedro no es tonto, sino inteligente. Juan is not stupid, but he is intelligent.
El culpable no soy yo sino tú. The guilty is not me but you.
Also, we already posted about another structure with “sino” (post 23 Abril):
No solo/no sólamente + SINO (QUE) + además/también (not only+but also): It’s used to affirm two actions or activities.
No solo trabajo en un restaurante sino que también estudio Medicina. Not only I work at a restaurant but also I study Medicine.
No solamente habla español sino también inglés y alemán. Not only he can speak Spanish but also he speaks English and German.
María no solamente es inteligente sino que también es agradable. Maria is not only intelligent but also she is nice.