Usage Note
Pero is the standard adversative conjunction contrasting two clauses, while sino (also in this set) is used after a negative to introduce a replacement ('not X, but rather Y'). The distinction is strict: no es alto, pero es fuerte 'he isn't tall, but he is strong' (simple contrast) vs no es alto, sino bajo 'he isn't tall, but rather short' (correction). Pero can open a sentence for rhetorical emphasis in speech.
Examples
"Es difícil, pero posible."
Natural Translation
It's difficult, but possible.

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