In most cases, avoid the passive voice ("Jim is being driven to distraction by his hamster") in favor of the more concise active voice ("Jim's hamster is driving him to distraction"). A sentence is more effective when it centers on a subject that is doing something, rather than a subject that is being acted upon. Writers often use the passive voice to avoid "I," because they have been told from an early age that "I" is unacceptable. Different academic disciplines, and different instructors within those disciplines, have varying attitudes to the use of the personal pronoun: you would be wise to check. However, the sparing use of "I" can be an alternative to unwieldy passives like some of those cited above: "I think . . ." is both more effective stylistically and more honest than "It is thought that . . ." The passive is often used to avoid responsibility: "The economy of this Province has been mismanaged" is less incriminating than "We have mismanaged the economy of this Province."
When we write in active voice, we connect the subject of the sentence with the action it is initiating. That means that readers can clearly see who is responsible for the action in a sentence.
"Richard fired Archie."
Initiator action recipient.
That’s the active voice: the subject initiates an action that affects the recipient.
In passive voice, the recipient becomes the subject, and the initiator of the action sneaks away to the prepositional phrase at the end of the sentence. Now all the attention is on the recipient, who becomes the grammatical subject.
"Archie was fired by Richard."
Recipient action initiator.
These are simple sentences, but you can use passive and active voice in more complex constructions.
"In 1973, President Nixon fired Watergate special prosecutor Archibald Cox, who was close to indicting the President."
We can clearly see who did the firing in this sentence—Nixon. That’s active voice.
Now compare the passive version: