Active Voice vs. Passive Voice - Why do we use them?

In a lot of ways, non-native speakers understand English grammar a lot better than native speakers do.  Active and Passive voice is one of those things that most native speakers can use well, but can't really explain well.  On the other hand, a lot of students can explain it quite well.  Students can even do all the exercises to transform one to the other, but in the end they often can't use it naturally.

Here is a little explanation of each.

Active voice:

Subject A does something to Subject B

I fed my cat

Passive voice:

Subject B receives an action (from Subject A)

My cat was fed (by me)

So we can make sentences like "I hit my brother" or "my brother was hit", and they basically mean the same thing.  A lot of classes spend a TON of time practicing changing from active to passive and passive to active, but unfortunately not so much time understanding why you would use one or the other.  

A lot of native speakers are told to just not use the passive voice (I remember being penalized for using the passive voice on a paper once...).  However there are good situations to use the passive voice, it's just that we tend to use the active voice more.

3 reasons why we use the active voice:

• Active voice is mainly spoken 
• Active voice is emotional
• Active voice has blame or responsibility (he did it!)

3 reasons why we use the passive voice:

• Passive voice is mainly written
• Passive voice is informational
• Passive voice doesn't know or doesn't care who did it

Finally, just a few examples of natural situations for each.  Active voice is often used in conversations, and generally most of the time.  Passive voice is trickier.  Passive voice is good in scientific writing, news stories, and when you don't care or don't know who did something.