Lifting the Front Foot #1
Lifting your front foot on your back swing (1) spells disaster for the rest of your swing. From lifting your head, to raising your torso and finally turning your front foot on the down swing and changing the direction of your target line. This is the first way you can resolve the situation.
If you bring in your knees as shown in the bottom picture, you will not be able to lift your front foot.
3) Weight Distribution
If you bring in your knees you will then feel the weight brought to the inside of your feet which will allow for an easier movement to the back foot on the take away.
The easiest way to see if you moved your front foot during your swing, check the angle of your front foot when you start your swing and then take a look after your swing. If your foot is more open, you lifted your foot and the ball just went left or right.