If your cars battery is good and your car is not starting then you could have loose battery terminal connections or the corrosion on the battery terminals may be making your battery terminal connections lose contact.
Corroded battery terminals can cause a car to fail to start if the battery is fully charged and the starter is just clicking or the battery which is good goes dead even after driving.
Corrosion on battery terminals can also make the battery unable to charge properly.
However if the car is cranking over but still not starting then it could be low fuel pressure, bad ignition coil, bad fuel pump, bad spark plugs, bad ignition control module etc.
It's hard to say exactly what could be causing your car to fail to start without knowing what it is actually doing or what the make and model of the vehicle is.
Corroded batteries are not dangerous but you should still avoid touching the battery corrosion with your bare hands.
If you need to clean battery corrosion you should wear gloves and use some baking soda and warm water to clean the battery corrosion off.
When your cars battery has corrosion you can mix some baking soda and warm water together and then pour the baking soda and water mixture onto the corrosion on the car battery.
Allow it to work for around 5 to 10 minutes.
Then use a wire brush to help brush off the battery corrosion and then use a hose to wash the corrosion away.
Battery corrosion can make a battery seem like it's dead because it can prevent charging of the battery through the alternator and also make a bad contact where the starter may or may not engage.