It looks like it's going to do me some good to review the basics of grammar and style. Yesterday, I confidently suggested that the following sentence is in the passive voice.
It is the number of writing sessions, not their length, that determines how much writing you get done.
Fortunately, Jonathan spotted the mistake and pointed it out, and this sent me back to the Chicago Manual of Style to relearn what I thought I already understood.
The passive voice is covered in §5.112. First lesson: voice isn't a property of sentences but of verbs. The voice of the verb determines whether the subject acts or is acted on. If the subject acts, i.e., carries out the action of the verb, then the verb is in the active voice. The following is in the active:
The number of writing sessions determines how much writing you get done.
And the following is in the passive:
How much writing you get done is determined by the number of writing sessions.
The question is whether the subject (the number of writing sessions in the first example and how much writing you get done in the second) is acting (determining) on the object (how much writing you get done in the first and the number of writing sessions in the second) or being acted on (being determined by) it.
Second, as Jonathan also reminds us, only transitive verbs have voice. So while it is true that passive voice involves the verb to be (it is "formed by joining an inflected from of to be ... with the verb's past participle," as the CMS puts it), non-transitive uses of to be don't have any voice at all. "It is raining," for example, is not passive. Nor was my sentence about the number of writing sessions.
The main reason not to use the passive voice is that it has a tendency to obscure the identity of the agent, i.e., the thing or person that is acting. That isn't really a problem in the examples we've been looking at. We can approximate such a problem by starting with the following sentence in the passive voice:
The quantity of writing was determined.
This is grammatical but not informative. How do we make it active?
... determined the quantity of writing.
The only way to make it active and grammatical is to introduce a specific subject that was not mentioned in the passive version. That's normally what is meant when you are being critized (oops, what I mean when I criticize you) for using the passive voice. The active voice normally forces you to consider the question of who or what is doing what to whom or what.