The single water knob to my tub is extremely hard to pull/push. How can I fix this?

Turn house water off, pull knob out all the way, remove knob, lightly coat the pulled out shaft with plumbers grease. Put the knob back on and work it a few times. Turn water back on. If you don't have plumbers grease, use white petroleum grease. You can also disassemble the valve and grease it but plumbers grease works almost all the time.