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).
Making meals for one often leaves you with leftover ingredients that will eventually go bad in your fridge. [Globe and Mail]
Ms. Donley also warns parents to discard perishable leftovers. [New York Times]
In mere minutes, you can whip up a rainbow bouquet from scraps of your leftover yarn. [Guardian]
Wilde Lake has plenty left over from its championship run. [Washington Post]
Dozens of Stinger missiles, left over from the 1980s anti-Soviet resistance, are thought to remain hidden. [Telegraph]
All they want to do is make a living, pay staff and hopefully have a few dollars left over. [Herald Sun]