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Looking for information from tax professionals about filing an amended return

I am looking for information from tax professionals on filing an amended tax return. In particular, I am interested in finding out: 1. When would you need to file an amended tax return? 2. Are there any penalties for filing an amended return? 3. What if you do not file an amended return when you are supposed to? 4. How can you follow up with the status of an amended return? Thank You! Requirements: Tax experts.