In English, when the subject of the verb is performing the action, then the sentence is written in the active voice as in "The dog bit the cat". The sentence "The cat was bitten by the dog" uses the passive voice. Can you see the difference? In the first example, the dog is active. It is the one who bites. In the second sentence the dog is still active - it is still the one who bites. The subject of the second sentence, however, is the cat. The cat is passive because it is a recipient of the action.
We could change this, of course. If we wanted the cat to have a more active role in the sentence, we could write, "The cat hissed viciously when the dog bit it." Becoming a good writer involves thinking carefully about such details when you construct your sentence. If you want the audience to picture the scene focussing on the dog, then an active construction where the dog is the subject is best. Writing "the cat was bitten by the dog" takes the emphasis off the dog (which now comes at the end of the sentence), but also leaves the cat in a passive role.
One common problem with passive sentences is that they do not always make it clear WHO is acting. This can be useful for formal, impersonal instructions, however, as you will see in this quiz.
If you get top marks, you are doing really well!