Leftover vs. left over | Grammarist | Spelling The one-word, unhyphenated leftover is usually an adjective preceding the noun it modifies (e.g., leftover pasta). When it follows the noun it modifies, left over is usually two words (e.g., there’s some pasta left over). Leftover also works as a noun, usually pluralized (e.g., save the leftovers).

In conclusion, "leftover" and "left over" are two different phrases with the same meaning. "Leftover" can be used as an adjective or noun and "left over" is a two-word phrase, used as an adverbial phrase. Both can be used interchangeably, but "leftover" is considered more colloquial and informal, while "left over" is considered more formal.