Question

The boy's mouth opneed. (Use mouth as an object )
Answered my question

Answer 1

the boy opened his mouth

Answer 2

The boy opened his mouth.

The object in any sentence is considered as the entity which is acted upon by a subject.

There is hence a major distinction between objects and subjects which is understood properly on behalf of action which is expressed by verb.

The traditional sentence structure theories divide the sentences into a predicate as well as a subject.