What type of energy always stays the same?
There are chemical, electric, magnetic, gravitational, nuclear, and thermal (kinetic).
All can be changed for the other. I can not think of an exception.
Your teacher must have something specific in mind, I have no idea.
The concept of energy conservation is what you might be referring to. According to the principle of energy conservation, the total amount of energy in a closed system remains constant. In other words, energy cannot be created or destroyed, but it can only be converted from one form to another.
For example, when a pendulum swings back and forth, it continuously converts its potential energy (due to its elevated position) into kinetic energy (due to its motion), and vice versa. However, the total energy of the system (potential energy + kinetic energy) remains constant.
In a closed system where no external forces or energy inputs are present, the total energy of that system will always remain the same. So, in this sense, the answer to your question would be that the total energy of a closed system always stays the same.
It's important to note that while the total energy remains constant, the distribution of energy among different forms can change. The ability of energy to transform from one form to another is what allows us to harness and utilize various types of energy for our needs.
It's always a good idea to clarify with your teacher if there is a specific context or aspect they are referring to when asking questions about energy.