The main difference between the two feet is that the standard foot, when lowered, "clamps"the fabric to the feed dogs so that they can move the fabric with each stitch. The darning foot or FMQ foot is slightly raised from the fabric (at least the one on my 401 is, which is my only experience) so that the fabric can be moved. To use the standard foot, you would need to reduce the pressure on the foot so that you can move the fabric. You would also have to disable your feed dogs by lowering, covering them or setting your stitch length to 0.
bkay