"It's a nice day out today."
"Yes, but...the sky is very blue."
-- or --
"Gonzalo works as an engineer."
"Yes, but, he speaks English."
BUT is a conjunction that shows contrast. In Spanish, pero can be used to bring together ideas. In English, BUT is not used to connect ideas: it's used to show that one thing is not the same as another.
- He owns a really nice house but he doesn't have a lot of money in his bank account.
- He's allergic to peanuts but he really likes peanut butter.
- Tyson loves the movies but he rarely goes to the cinema.
If you use BUT too much when you talk, it can sound like you are continually disagreeing with the other person.