(6)  What is a passive and why should you care?
Passive clauses in English are paraphrases of active, transitive clauses.  In other words, a clause like The cats chased the rats (which is active and transitive) has a paraphrase The rats were chased by the cats; similarly the active A spy has seen me has a passive I have been seen by a spy.  There are several properties that distinguish passive clauses from active ones:
  1. There is always a form of be (be, been, being, am, is, are, was, were) as the verb immediately before the main verb (in these cases, chased and seen).
  2. The subject of the passive is the same as the first object of the active (in these cases, the rats and I (me)).
  3. The subject of the active can appear as the object of the preposition by in the passive.
  4. The main verb is always a past participle in the passive (in these cases, chased and seen).

A passive clause like I was hit by a car or Ozzie has been treated for his illness implies the existence of an active clause like A car hit me or Someone has treated Ozzie for his illness.  If you cannot find an appropriate active in which the subject of the passive occurs as an object, your passive is probably not a good sentence.  So Harriet has been happened to get ill is ungrammatical, since there is no grammatical sentence (Something/Someone) has happened Harriet to get ill.

In general, you want to avoid using passives unless

  1. you are quite confident that there is an appropriate passive, and
  2. you have an appropriate rhetorical reason to use it.
The primary rhetorical reasons to use a passive include
  1. avoiding mention of the subject of the active or decreasing the prominence of the subject of the active;
  2. increasing the prominence of the object of the active; and
  3. making the event itself less active.
If you want to avoid mention of the subject of the active, converting the clause into a passive and omitting the by phrase works admirably.  In this way, I was attacked allows the speaker to talk about the event without overt mention of the attacker.  You can avoid identifying the attacker by using an indefinite pronoun (or other indefinite noun phrase) as in Someone attacked me or Some idiot attacked me.  However, both of those imply that the speaker does not know the identity of the attacker.  If you merely want to avoid mention of the attacker, a passive clause is appropriate.  Similarly,  Bears attacked me suggests greater focus on bears than does I was attacked by bears and a concomitantly lesser focus on I (me).

Active Full Passive Agentless Passive
Charley attacked Bill. Bill was attacked by Charley. Bill was attacked.
The teacher gave the children some candy. The children were given some candy by the teacher. The children were given some candy.
Everyone considered Mary a genius. Mary was considered a genius by everybody. Mary was considered a genius.
Somebody is watching me. I am being watched by somebody. I am being watched.
They have bothered her. She has been bothered by them. She has been bothered.
Has this helped you? Have you been helped by this? Have you been helped?

Why should you care?
    First, because this is a construction non-native speakers often get wrong.  They try passives of verbs which are not transitive, for example.   The victim was killed is a perfectly fine passive (kill is a verb that takes an object), but *The victim was died is not (die is not a verb that takes an object).
     Second, because even if the passive might be grammatical, it might not be rhetorically appropriate.
     Third, because it is a technical term which lots of teachers use (sometimes appropriately, sometimes not) and if they are objecting to something about your writing, it is preferrable to know what that something is.
To read more about this topic, look at Elizabeth Siler's "Understanding the Active and Passive".