2004 Ford Focus Starting Problems

Engine is having problems turning on. Spark plugs have been changed and no difference. It's a zts 2.3 liter model, so it has no plug wires. Negative battery terminal had corrosion, I cleaned it off and still no difference. Changed the fuel filter and it starts a little bit better now but still not as it should. Could it be the fuel pump? What happens is that as the car is trying to start, the engine keeps turning like normal, but it seems like the fuel is not reaching the cylinders properly so it takes longer to turn on. Have been running the engine on low fuel, less than a quarter tank until it is almost empty. Not been letting the engine warm up either.