leaving on the radio
mainly leaving your lights on when the engine is of or leaving the keys in the ignition
No, not unless the ignition is turned on.
If the engine is running, then no. The alternator constantly keeps the battery charged.
Yes, bu very slowly. Will cause dead battery in about 1 month
No, the charger pulls so very little power as to not be an issue unless the car was not run for a long period of time. It would take a very long time for the charger to drain the battery.
Leaving the car stereo on overnight can drain the battery and make the car impossible to start in the morning.
The battery has only so much energy available. Headlights use a lot of energy. If you leave them on for any amount of time with the engine not running, less energy will be left in the battery to turn the starter. This is normal and to be expected. The older the battery, the less energy it can store. So if the battery in your car is old, it will make the problem worse.
If you are out of gas or something and keep cranking the starter then yes sooner or later you are going to drain your battery. Age of the battery is a factor, the older the battery the less time you get. Also the temperature if it is below zero out you get a lot less time.
A car battery in any car would drain due to a short in the electrical system.
It will probably keep transmitting and place a small drain on the battery.
Parasitic voltage drain. Are you leaving something like an inverter plugged in? If you installed a new car stereo or amp for example and connected to a power source that is "hot" all the time, whatever is on that line will drain the battery all the time.