Answer to Question 1
Answer: You are using the active voice when the subject performs the action, and the object receives the action: John rented the office. You are using the passive voice when the subject receives the action: The office was rented by John.
Answer to Question 2
Answer: The passive voice can be helpful when you need to shift emphasis away from the person who is the subject of your message. These situations include (1) when you need to be diplomatic in pointing out a problem or error, (2) when you want to point out what's being done without taking or attributing either the credit or the blame, and (3) when you want to avoid personal pronouns in order to create an objective tone.