The old adage that the left arm should always be kept straight is still as relevant as ever – as long as you interpret it correctly.
It does not mean that you must keep the arm absolutely ramrod straight and stiff, because that defeats everything. The object of the golf swing is to create the widest possible arc. That is impossible if the left arm is bent at any point in the backswing or downswing, and will result in a chopping action.

Maintain a wide arc by turning your shoulders and keeping your left arm as straight as possible. Then as you move into the down swing, your weight shifts onto your left leg and your arm should be pulling along the line of the shaft.