Pendulum clocks are usually antiques or heirlooms kept in the family for many years, so you want to keep the clock running in good condition. Pendulum clocks should be lubricated every three years and taken completely apart and cleaned every six years. In between this maintenance, there are some problems you can attempt to address yourself.
Check the beat of the clock. If the beat between the swings of the pendulum sounds off, the clock may not be level. Tilt it one way then the other to see if that fixes the beat. If that doesn't work, prop one side or the other up.
Check the height and speed of the pendulum. To make the pendulum swing faster, raise the bob (the round part at the end of the pendulum). To slow the pendulum, lower the bob.
Check and set the correct time. You can only move the minute hand clockwise. In moving that hand, it will come around and catch the hour hand and start keeping the correct time.
Check the bell. If the bell sounds flat and doesn't ring clearly, it may be loose. Check the nut and screw holding the bell to make sure they are tight. Make sure the hammer arm is about 1/16" away from the bell.
Find a professional clockmaker if your clock is still not running properly.