What I did:
I went online to look up on active voice and passive voice
What I learned:
Sentences in the active voice have the subjects doing or "acting upon" the verb in the sentence.
The man (subject) have just eaten (verb) 2 hamburgers
Mary (subject) mailed (verb) the letter
In the contrary, when the verb "acts upon" the subject, the sentence becomes passive and this can be achieved by changing the normal word order of active sentences with a direct object, so that the subject is no longer active.
2 hamburgers have just been eaten (verb) by the man (subject).
The letter was mailed (verb) by Mary (subject).