Aside from not properly explaining what passive voice is in the first place, I don't think any of my English teachers ever explained why we supposed to avoid it, beyond "because I said so", or, at best, some vague admonitions about it making your writing weaker. Overuse of anything can make writing feel repetitive and stale, certainly, but avoiding something entirely, and with no real explanation as to why? That sort of thing never sits well with me. If I'm expected to do, or not do, something, I at least want to understand why. Even when I don't necessarily agree with the reasons, it's helpful to know that there are reasons. And, as should go without saying at this point, I can't remember anyone ever mentioning any of the various reasons why you would want to use passive voice.
So, why don't we start with what is and is not passive voice? The verb "be" has three major uses in modern English (unless I'm overlooking one), and only one of them puts a phrase in passive voice.
As a primary verb to indicate state of being
This is probably the most common usage of "be". Let's start with some examples, emphasizing the (entire) verb in each case.
- I am the writer of this blog entry.
- The seaweed is always greener in somebody else's lake.
- People are often stubborn.
- It was a dark and stormy night.
- The chickens were restless.
- This could be the discovery of the century!
- While everyone else raced around, the meditating stranger was simply being.
- It had been a strange series of events.
This form of "be" either acts as a synonym of "exist" or, more often, links a subject to a description or identification, similar to a verbal equal sign. You could make the case that this isn't exactly active voice, since nothing is actually doing anything, but it's not passive voice, since nothing is happening to anything, either.
Sometimes, there are effective ways to rephrase a sentence to avoid using a form of "be", and it couldn't hurt to take a look at sentences like these and see whether it makes sense to rewrite them. For example, "the grass in the meadow was green" could become "green grass grew in the meadow". When it works, this can help writing sound more vibrant. In many cases, though, it's more trouble than it's worth and just makes the result sound unnatural. Unless you want unnatural, that's not a good thing. Try to avoid overuse, yes, but realize that descriptive sentences with no actions have their place.
As an auxiliary verb to form progressive tense
- I am writing this blog entry.
- It is raining.
- The peasants are rioting in the streets.
- I was eating dinner when the phone call came.
- They were calling her name.
- You should be finishing your homework.
- (If there's any natural way to use a "being [verb]ing" phrase that makes sense, I can't think of it.)
- The government bureaucrats had not been doing anything useful for years.
Note that the verb in each of these cases indicates an action, and that the subject of the sentence is taking (or neglecting to take) that action. That's the very definition of active voice. You should only avoid sentences like these when the progressive tense itself does not correctly describe the action, never because of some phantom rule about avoiding "be". The only way a sentence with a verb in "-ing" form after a form of "be" can even possibly be passive is when the "-ing" verb is "being", and even then it's not a sure thing, as the example sentence in the first section above demonstrates.
As an auxiliary verb to form passive tense
- I am dismayed by the confusion about the passive voice.
- Seafood is commonly eaten in coastal regions.
- All trains are delayed due to the snowstorm.
- She was enthralled by the possibilities of the new invention.
- The adventurers were defeated in combat.
- Sleep can only be avoided for so long.
- Some people are being fangoriously devoured by a gelatinous monster.
- The new tech had been hired straight out of high school.
As above, the verb in each of these cases indicates an action, but unlike above, the subject of the sentence is not taking the action. Instead, it happens to the subject. If you see something that looks like "[verb]ed by", you're almost certainly looking at the passive tense, but note that "by" does not need to appear. When it does, however, you can rephrase a passive voice sentence "X was verbed by Y" as "Y verbed X"—but English teachers aside, you may not always want to do that.
Passive voice exists for a reason. Sometimes you don't know who performed the action, just who it affected. Sometimes you don't care who did it. Even when you do know and (at least to some extent) care, that information might be incidental to the thrust of the sentence. Passive voice works perfectly when you want to emphasize the consequences of an action, or to focus on its recipient rather than its performer. If you're telling a story about Arthur, then saying "Arthur was mugged" instead of "someone mugged Arthur" is not only perfectly fine but makes more sense. It's Arthur's story, not the mugger's.
Incidentally, you can make a passive verb without using any form of "be" at all, though it may not be considered proper English. If robbers "got chased off", for example, that's passive, since someone else chased them off, and it happened to them.
In a typically English tendency to make things more confusing, some words can be either the past participle of a verb or an adjective. This makes it difficult in some cases to tell whether a given sentence is using passive voice or whether the "be" acts as a connective and the following word acts as an adjective instead. For example, if the water "was frozen", "frozen" could be the past participle of "freeze", making this a passive verb, or it could be an adjective for something cold and solid, which would make this a descriptive sentence not in passive voice. Personally, I say leave the distinction to the linguists, and just use whatever works without worrying overly much about what it is grammatically.
And one more...
"Be" once had another usage, but this one has mostly died out in modern English. The appropriate form of "be" before the appropriate form of another verb indicated an action that occurred in the past and resulted in a state that continues to the present. Possibly the most familiar usage of this form comes in religious hymns. "Christ is born", "He is risen", and so on all indicate that something happened with a result that remains in effect. Similarly, someone who "is fled" has run away and not returned, more explicitly than someone who simply "has fled". Once again, this is not in any way passive voice.
"Be" is not your enemy. It doesn't always mean passive voice, and even when it does, that isn't automatically a bad thing. Don't overuse either the passive or "be", but don't be afraid of them either. They have their uses, and understanding them will help, not hinder, your writing.