The words morn, mourn sound the same but have different meanings and spellings. Why do morn, mourn sound the same even though they are completely different words?
The answer is simple: morn, mourn are homophones of the English language.
The morning.
The dawn.
To feel or express grief or sorrow. See Synonyms at grieve.
To show grief for a death by conventional signs, as by wearing black clothes.
To make a low, indistinct, mournful sound. Used especially of a dove.
To feel or express deep regret for: mourned the wasted years.
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.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").