TO consistently hit the ball straight and control its flight, your position at the top of the backswing and the transitional move from there into the downswing are key areas to note.
Essentially, the downswing path is a product of your position at the top and dictates your swing path through impact.
Indeed, the swing path influences both, where the ball starts and the angle of the clubface at impact.
Swinging the club along a neutral path and through impact helps start the ball on line. When the face is square to this path, the ball will not curve in the air. To check if you are swinging on a neutral plane, the shaft should bisect the right bicep when your left arm is parallel with the ground.
When the club is flatter it promotes an in-to-out swing path that starts the ball right. You can check this when the lead arm is parallel to the ground. This time the shaft runs through or even below the right or trail elbow. If the clubface is closed to the in-to-out swing path the ball will draw or hook.
This time the club is steeper. It promotes an out-to-in swing path that tends to start the ball left. If the shaft angle runs above your trail shoulder at this stage, this is your swing pattern. If the clubface is open to the out-to-in swing path, the ball will fade or slice.