Why car wont start if the headlights are on?

most likely cause is the alternator not charging correctly. normal charge voltage with all accesories on is around 13 to 14 volts.anything less than 13 volts means that your alternator isn't charging and the car runs off of the battery until it can't run anymore. As the voltage drops, there is less power to produce a spark and the spark gets weaker or disappears until it can no longer ingnite the fuel/air mixture. You will probably notice that the top speed you can go keeps dropping as the battery drains. When it stalls you will not be able to get it to crank again without charging it.