The difference between the active and passive voice

Voice shows the relation of the subject of the sentence to the action of the verb.

In the active voice, the subject is the agent (or doer) of the action:

Alexander Fleming discovered penicillin in 1928.

With the passive voice, the subject is the recipient of the action. It is the person or thing that the action is directed at:

Penicillin was discovered by Alexander Fleming in 1928.

The starting point of a sentence is usually the subject. It provides the theme of the message, which is old information, while new information is supplied near the end of the sentence.

With the active voice, the theme is the agent. We say something about the doer of the action, who or which is the subject of the sentence:

Alexander Fleming was a Scottish biologist, pharmacologist and botanist. He discovered penicillin in 1928.

With the passive voice, the theme, or old information, is the recipient of the action. We say something about the recipient, who or which is the subject of the sentence:

Penicillin is an antibiotic that kills most bacteria. It was discovered by Alexander Fleming in 1928.