- A torpedo sunk the ship.
- The ship was sunk.
- The ship sunk.
The first is active, the second is passive.
http://www.plainlanguage.gov/howto/quickreference/dash/dashactive.cfm contains the frequently-cited rule:
When we write in the passive voice, we add some form of the helping verb "to be" (am, is, are, was, were, being, or been) to an otherwise strong verb that really did not need help.
But what about number three? A debater could take either side:
- Active: the ship performed an action upon itself, as if sinking itself were analogous to sinking another ship
- Passive: The action happened to the ship as a result of some unspecified circumstance, identical in meaning to number two.
But I don't want to debate, I want to know which is the right answer :-).