Joining contrasting ideas with が

After a statement, が can mean “but.” この 車くるま は 高たかい です が、いい です means “This car is expensive, but it’s good.” Finish the first idea, add が, then give the contrast.

Look at what comes before it. In 猫ねこ が います, が marks the subject. In 高たかい です が, it follows a statement and connects it to what follows. The same particle form has different grammatical jobs.

Connecting が can also introduce background or soften what follows, so it does not always imply a sharp contradiction. A speaker may trail off with 行いきたい です が…—“I’d like to go, but…”—leaving the rest for the listener to infer.