Complex sentence: There are many forms of energy, but all these types must meet certain conditions such as being convertible to other kinds of energy, obeying conservation of energy, and causing a proportional change in mass in objects that possess it.
Split into simpler sentences: There are many forms of energy, but all types of energy must meet certain conditions. In order to be valid, it must be convertible to other kinds of energy, obey conservation of energy, and cause a proportional change in mass in objects that possess it.