Energy is not conserved in non-linear systems.

Virtual work theorem is applicable to non-linear systems.

Virtual work theorem is applicable to non-conservative systems.

Kinematic admissible functions must satisfy boundary conditions on forces and moments.

Statically admissible functions satisfy equilibrium equations.

Degree of static redundancy is zero for statically determinate structures.

Virtual displacement must satisfy equilibrium equations.

Virtual displacement must be kinematically admissible.

If virtual work due to kinematically admissible displacements is zero then the displacements satisfy equilibrium.

If virtual work due to statically admissible forces and moments is zero then displacement boundary conditions are met.