If a car is hard to start, the issue is often the battery. While a dead battery does not turn the starter at all, a weak one turns it very slowly. This slow turn does not create enough compression to cause the fuel to ignite in the cylinders, making it difficult to start the vehicle.
Corrosion around the battery terminals sometimes causes the starter to drag so the engine does not start. O'Reilly Auto Parts suggests cleaning the terminals using baking soda, water and a stiff brush. For safety, the cables should be removed before cleaning the terminals, beginning with the negative one first.
During cold weather, gasoline vaporizes slowly. Since it must vaporize to burn, the vehicle is more difficult to start. During extreme cold, some drivers spray ether starting fluid into the vehicle's fuel system. Ether has a lower vapor pressure and burns at a lower temperature than gasoline. This creates enough heat that the gasoline begins to vaporize and the engine continues to operate.
Motor oil becomes thicker and more difficult to move during cold weather. This makes it more difficult for the starter to turn the engine. One of the advantages of synthetic oils is they remain fluid in cold temperatures.