When your sentence uses an active voice, the subject is doing the action. Here's an example:
John loves oranges.
'John' is subject and 'oranges' is the object
If your sentence uses a passive voice, the object is now the new emphasis. Let's take our example and turn it into a passive voice:
Oranges are loved by John.
Here the subject has transferred to 'oranges', but they aren't necessarily doing anything. They are simply a receiver of John's love. The focus of this sentence is now on 'oranges'.
Here are some more examples:
Passive: The window was broken by her.
Active: She broke the window.
Passive: Many experiments were conducted by the scientists.
Active: The scientists conducted many experiments.
Passive: The drums are hardly played by Anna.
Active: Anna hardly played the drums.
Changing your sentences from passive to active voice can be useful when trying to be clear to your reader. An argumentative paper, for example, can be more effective when written in the active voice. See the following example:
Passive: Several things are indicated by these results.
Active: These results indicate several things.
Here, the active sentence provides a smoother transition between the information previously presented in the essay and what is going to be presented next.
When is Passive Appropriate?
There are cases where the passive voice can be appropriate. It can be used for effect or to be more persuasive. The following example indicates a passive sentence in which a politician is trying to emphasize the need for cotes:
Passive: 100 votes will pass this bill.
This passive voice is appropriate because the speaker wants to put emphasis on the votes rather than the bill. If the sentence was written in active voice (The bill will be passed with 100 votes), the emphasis would no longer be on the votes.