"But" is the same as "however." Whatever follows the word "but" goes against what is in front of the word. It is also usually more important. Joe wants to travel but he has no money. This gives you two statements that are both facts. The first part, before the "but," tells me that Joe wants to travel. The second part, after the "but," tells me that Joe can't travel because he doesn't have money. Daniel feels sad for the animals in the zoo, but lots of them were sick in the wild and now the zookeepers look after them. The first part, before the "but," tells me that Daniel wants the animals to be free from the zoo. The second part, after the "but," tells me that he also thinks the animals are healthier in the zoo.