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Active voice is usual the best way to write, as it sounds smoother and logically has the doer of the action as the subject. You can recognize passive-voice expressions because the verb phrase will always include a form of to be, such as am, is, was, were, are, or been. The presence of a be-verb, however, does not necessarily mean that the sentence is in passive voice. In sentences written in passive voice, the subject receives the action expressed in the verb; the subject is acted upon. In sentences written in active voice, the subject performs the action expressed in the verb; the subject acts.
Here are the notes from class:
Listen to the Grammar Girl episode on active and passive voice.
The homework exercises: 
Active vs Passive
