The words warn, worn sound the same but have different meanings and spellings. Why do warn, worn sound the same even though they are completely different words?
The answer is simple: warn, worn are homophones of the English language.
To make aware in advance of actual or potential harm, danger, or evil.
To admonish as to action or manners.
To notify (a person) to go or stay away: warned them off the posted property.
To notify or apprise in advance: They called and warned me that they might be delayed.
Past participle of wear.
Affected by wear or use.
Impaired or damaged by wear or use: the worn pockets on a jacket.
Showing the wearing effects of overwork, care, worry, or suffering.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.