Why will an engine not start but will turn over?

Is there gas in it?

Fuel tank empty . Fault in the carburetor and fuel injection system . Battery terminals connections loose or corroded , battery discharged ( eng rotates slowly ) . Fuel pump faulty . Excessive moisture on , or damage to ignition components . Worn, faulty or incorrectly gapped spark plugs , Broken ,loose or disconnected wiring in the starting circuit , ignition coil or faulty coil . Low cylinder compression , Faulty distributor pick-up coil or ignition module ?