The words gnu, knew, new, nu sound the same but have different meanings and spellings. Why do gnu, knew, new, nu sound the same even though they are completely different words?
The answer is simple: gnu, knew, new, nu are homophones of the English language.
Either of two large African antelopes (Connochaetes gnou or C. taurinus) having a drooping mane and beard, a long tufted tail, and curved horns in both sexes. Also called wildebeest.
Past tense of know.
Having been made or come into being only a short time ago; recent: a new law.
Still fresh: a new coat of paint.
Never used or worn before now: a new car; a new hat.
Just found, discovered, or learned: new information.
The 13th letter of the Greek alphabet. See Table at alphabet.
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.