The left pedal screws on anti-clockwise rather than the usual clockwise. This is to ensure that the pedal cannot work loose as you exercise and stays tight. The right-hand pedal screws in clockwise as normal.
If you have tried to fit the left-hand pedal by turning it clockwise and find that having read this article it will not fit anti-clockwise either, then it's highly likely that the thread has been damaged while trying to fit the pedal on clockwise. If you find yourself in this position please use the contact form to let us know and we'll arrange for a replacement set of pedals to be dispatched to you free of charge.