First of all, we probably learned to swim freestyle by taking each movement (arms, legs, etc) separately. That may be ok for a beginner but a more advanced freestyler needs to balance the body by synergising all the various components into one smooth, coordinated whole.
Freestyle is probably the most complex stroke and, as any beginner will tell you, the most difficult task to achieve is coordination of all its components. Poor arm stroke, ineffective kicking, poor head position, uncoordinated breathing and lack of torso rotation will only add to drag, holding the body back and tiring the swimmer limiting their endurance.
Tips on improving your freestyle will appear here over the next few weeks.