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In addition to rewarding them when they do it right (outside) you absolutely MUST scold them when they go inside.

Dogs learn fairly quickly when punished. Next time your dog takes a piss on your carpet, put her nose in it and say "NOOO" forcefully. Hold her there and make her smell the mess for a good 5-10 seconds saying "no no no!" repeatedly..then release her. They get the message. This isn't cruel. It's how they learn. My black lab was trained in about 1 week by doing this. He now goes to the back door to let me know when he needs to go out. No accidents for several years now.