The interaction between the subject and verb of a sentence determines voice. Put another way, to figure out voice in a sentence, ask yourself “does the subject of the sentence perform the action or receive it?” If the subject performs the action, the sentence is active; if the subject receives, then the sentence is passive.
In an active-voice sentence, the subject of the verb is performing an action:
Margaret Atwood wrote The Handmaid’s Tale.
The researcher asks the subjects twenty simple questions.
A black car has hit my dog.
You should use active voice when you wish to emphasize the doer of the action.
In a passive-voice sentence, the subject of the verb is receiving an action:
The Handmaid’s Tale was written by Margaret Atwood.
The subjects are asked twenty simple questions.
My dog has been hit by a black car.
You should use passive voice when you wish to emphasize the receiver of the action.