Why is it advisable to complete your dose of medicine even though you are feeling better within a few days of the course of the medicine?

You must complete your dose because it helps in eliminating all the disease-causing microbes from the body. We might feel better after taking the medicine for a few days but shouldn't discontinue the course of medicine in between. The microorganism can develop resistance if we don't complete the dose given by a doctor. Due to resistance more and more harder drugs need to be manufactured to kill these microorganisms.

Why do we pull our hand when we touch a hot object? How does our body know it is hot?

There are thermoreceptors in our skin that detect the change in the temperature and send signals to the brain through nerves which send signals to muscles in hand to pull the hand away from the hot object. All of this happens within a fraction of a second.

Why is the earth not pulled into the sun?

It is because of the forces. The bodies experience a gravitational force which is in the balance and not the only sun attracts earth but other planets; heavenly bodies also do which balances the forces and the net force is zero.