the analytical postposition "a" is used on core arguments, the usage of the postpostion "a" depends on the existence of volition and the animacy of the noun it modifies.

when the agent of a transitive verb is an inanimate object, or when the agent of any verb is an unvolitional animate object(i.e. lack of control over the action, performs the action without the existence of volition, etc.), the postposition "a" is used;

when the patient is an animate object, the postposition "a" may be used or may not be used;

in other situations, the postposition "a" is not used.