Turn the headlights on, and watch their brightness while you try to start the motor. If the lights dim way down or go out when you turn the key to start; the battery is either discharged or the battery posts and cable clamps need to be scraped with a round file or a tapered reamer type battery cable service tool; until their mating surfaces are shiny. Reinstall the battery cables and tighten the clamp bolts until the clamps cannot be moved by hand. If the lights still dim and the starter does not run after cleaning the battery posts; have the battery charged with a battery charger and then tested with a LOAD TESTER (not just with a voltmeter or hydrometer).
If the headlights stay bright when you turn the key to start:
If you have an automatic transmission; try moving the shifter into neutral and see if it then starts. If you have a stick shift; be sure there is not a rug or thick mat under the clutch pedal which prevents it from being pressed all the way down when trying to start. Be sure to press the clutch pedal down until it hits bottom before starting.
If these actions still don't make the starter work; either the neutral safety switch (on automatic transmission vehicles), or the clutch switch (on manual transmission vehicles), or the starter relay or the starter solenoid is defective.
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