You can join two ideas with and when they go together. When the second idea is different, or a surprise, join them with but.
Key idea
Use but to join two ideas that are different, or when the second one is a surprise.
Why it works
But warns the reader: get ready, this next part goes the other way. “The box was big” makes you expect it to be heavy. “…but it was light” surprises you, and but tells you the surprise is coming.
Worked example: A surprise
“Leo is little. He runs fast.”
- You might expect a little boy to be slow.
- The second idea is a surprise.
- Join them with but.
Answer: “Leo is little, but he runs fast.”
Worked example: No surprise
“I like dogs. I have a dog.”
- The two ideas agree. Nothing is a surprise.
- So but does not fit. Use and.
Answer: “I like dogs, and I have a dog.”
Common mistake
Using but when the two ideas agree: “I like dogs, but I have a dog.” Ask: is the second idea different from what I would expect? If not, use and.
For the grown-up. Some writers and teachers say never to start a sentence with And or But. Careful writers do it often, so it is not an error; for now, practice joining two ideas in one sentence.