- Passive: The window was left open. (we are left wondering who left the window open)
- Active: Joe left the window open. (now we know that Joe is the one performing the action)
- Passive: The ball was kicked into the goal by Wendy. (Wendy is the one doing the kicking, but she isn't in the subject position in the sentence)
- Active: Wendy kicked the ball into the goal. (note that the active form of the sentence is shorter and more engaging)
The revision is slightly shorter and far more precise and gripping.
Again, the passive voice is not a grammatical error, and there are even times when you will want to use it. If you are trying to emphasize the object of a sentence, you may want to put it in the subject position in a sentence. For example, let's say a beautiful 300-year-old tree in your front yard was destroyed by lightning. If you write about the event, you probably want to emphasize the tree, not the lightning: "The old tree was destroyed by lightning last week." The sentence is passive, but appropriately so. The lightning may be performing the action (striking), but the tree is the sentence's focus.