Whenever I teach a grammar/writing class, invariably more than a few students (read: a majority) keep confusing passive voice with past tense. (I covered tenses in a previous Building Blocks post.)
English verbs can have just two voices–active and passive.
The active voice is what you employ for almost everything you say.
Active: "I am sitting at my desk typing a blog post."
Simple, straightforward English that depicts an action taking place–that’s the active voice.
Now, were I to make this into a passive voice sentence (I can’t imagine why I would, however), it would read something like this:
"A blog post is being typed by me while I am sitting at my desk."
Notice that the whole point of the passive voice is to turn the object (of an active voice sentence) into the subject. In this example, blog post has switched from being the direct object to being the subject. That’s why you end up with the passive voice verb construction, is being typed.
To make things even more ridiculous to the ear–and to the comprehension–to change the subject of the active sentence, I, to the object, I has to take the form of a prepositional phrase, by me. (And in this example, getting that "sitting at my desk" part in there is really cumbersome.)
I didn’t pick the greatest example, but here’s another.
Active: "I am eating a hamburger."
Passive: "The hamburger is being eaten by me."
The distinguishing feature between active and passive is that the passive construction actually starts with an object in a role reversal as a subject.
Bottom line: Avoid the passive voice as much as possible. Leave it to scientists ("the patient was observed to expire after three weeks of non-feeding") and crime scene investigators ("the body was found in a pool of blood").
English grammar was indeed found to be complicated when the student shouted in frustration, "English grammar sucks!"