Question

How does energy change with a swinging pendulum?

Answer 1

At maximum height, the pendulum has maximum potential energy `E_p=mgh` and zero kinetic energy.
As it falls it loses vertical height and so loses potential energy, but gains an equivalent amount of kinetic energy `E_k=1/2mv^2` by virtue of building up speed.
At the bottom most point, it has zero potential energy but maximum speed hence maximum kinetic energy.

At all times, the Principle of Conservation of Mechanical Energy applies in that the sum of `E_P+E_k` is always constant at all times during the pendulum motion.