A modifier is any word or phrase that modifies, or describes, another word.  Adjectives and adverbs are examples of single word modifiers, but entire phrases can also act as modifiers.  Consider these sentences (the modifiers are in bold):

Use the sharpest knife.   (sharpest describes/modifies the knife)

The snow was falling softly.  (softly describes/modifies the way the snow was falling)

Ask the girl with the curly hair.  (with the curly hair describes/modifies the girl)

The placement of modifiers affects the meaning of sentences.  “We only ate two slices of pizza” is different from “We ate only two slices of pizza.”  In the first example, only modifies what we ate: the two slices of pizza were all that we ate — no salad, no garlic knots, just pizza.  In the second example, only modifies the word two — we didn’t have three or four slices of pizza.  We stopped at two.   In general, modifiers should be placed as close as possible to the words they modify to avoid confusion.

A dog caught my attention running through the park.

In this sentence, it’s not clear who is doing the running — me, or the dog.  Was I jogging when I noticed the dog, or did the dog run by me while I was sitting on a park bench?  To clear up the confusion, I could say either, “A dog caught my attention while I was running through the park” (if I was the one doing the running) or ” A dog running through the park caught my attention” (if the dog was the one who ran).  Here’s another example:

Upon slamming the door, the vase fell from the table and shattered.

The wording of this sentence makes it sound as though the vase slammed the door, which seems fairly impossible.  Remember: place modifiers as close as possible to the words they modify: “Upon slamming the door, I saw the vase fall from the table and shatter” or “When I slammed the door, the vase fell from the table and shattered. “  Either way, I need to make clear that I am the one who did the slamming.

Returning to our original example, when Groucho Marx wonders how the elephant got in his pajamas, he is playing on the misplaced modifier: “I shot an elephant in my pajamas.”  We would assume that Groucho was the one wearing pajamas, but grammatically, the elephant was wearing them.

Active voice

In an active sentence, the subject performs the action:  “Gilbert studied his notes.”  Gilbert is the subject of the sentence and the action he performs is to study.  Another example:  “Lauren broke the pencil.”  Here, Lauren is the subject, and the action she performs is to break.

Passive voice

In a passive sentence, the subject receives the action: “The notes were studied by Gilbert.”  Even though the notes aren’t performing any action, the focus is on them rather than Gilbert, the one who is doing the studying.   Likewise, “The pencil was broken by Lauren” is also passive.  The pencil receives all the focus of the sentence, even though Lauren was the one who broke it.

Sentences written in passive voice are not technically incorrect, but they are not usually the most efficient way of wording a sentence.  “The notes were studied by Gilbert” is a much more clunky — and, consequently, confusing — way of describing what took place.

So should you avoid passive voice altogether?

You should only use the passive voice when you have a good reason for doing so.  In your lab reports, for example, you will probably find yourself using passive voice because you are meant to emphasize the process rather than yourself, the researcher.  As a result, your teacher will often prefer you to write, “The experiment was run four times” rather than “I ran the experiment four times.”

Another time it is appropriate to use passive voice is when you do not know who performed the action.  For example, “The mysterious letter was left on my doorstep” is correct because we do not know who left the letter.  Likewise, because passive voice de-emphasizes the subject, some people use passive voice to avoid taking responsibility for their actions — “Mistakes were made” (Well, who made them?) or “The paper was poorly written” — (Yes, but didn’t you write it?)

In the end, like most matters of style, the choice is yours to make.  Just remember to have intention behind each word of your writing — use passive voice with purpose, not because you couldn’t think of any other way to say it.