We describe here a special subdirect decomposition of algebras with modular congruence lattice. Such a decomposition is based on the properties of the congruence lattices of algebras. We consider four properties of lattices: atomic, atomless, locally uniform and anti-uniform. In effect, we describe a star-decomposition of a given algebra with modular congruence lattice into two or three parts associated to these properties.

We investigate first order sentences valid in completions of a given partial algebraic structure - a partial model. We give semantic and syntactic description of the set of all sentences valid in every completion of the given partial model - first order theory of this model.