Question

How do I know if a sentence is in the active or passive voice?

Answer 1

Answer:

Remember: If the subject is performing the action, then the sentence is in active voice. If the subject is simply receiving the action, then the sentence is in passive voice. Shift the object of the action into the beginning of the sentence.  

Example:

You did not pay your bill.

Answer 2

(for example)
Active - The dog chased the ball.
Passive - The ball was chased by the dog.

in a passive sentence the subject is acted upon by the verb, whereas an active sentence is when the subject acts upon a verb.
Active is more common than passive.