Question

under what conditions is mechanical energy conserved?

only when conservative forces are present

Only when the net talk on a system is zero only when the net force on the system is zero mechanical Everly is always conserved

Answer 1

Mechanical energy of system is the sum of all the energies of the system like potential energy, kinetic energy, thermal energy etc., Sum of different energies of a system always remains constant. There may be conversion of one form of energy to the other but total mechanical energy of the system remains constant.

Hence mechanical energy is always conserved.