When a sentence starts with a modifying phrase, readers expect that phrase to describe the first noun in the sentence. If that first noun is not the agent the opening phrase ought to modify, then the phrase becomes a dangling modifier.
A dangling modifier changes a sentence’s literal meaning into something the author didn’t intend. The results are usually illogical and often amusing.
Staring at the stunning blonde on the sidewalk, Jenny’s car ran smack into the lamp post.
At the age of sixteen, my parents got me a computer.
After reading the whole book carefully, the argument did not convince me.
In the first sentence, the car is staring at the blonde. In the second, your parents bought the computer when they were sixteen years old. The third sentence says that the argument read the book carefully.
While readers can usually figure out what the dangling modifier is supposed to mean (as opposed to what it literally says), a good writer never makes readers do unnecessary work.
How to Correct Dangling Modifiers
There are two simple ways to fix a dangling modifier.
1) Make the agent the subject of the sentence:
Staring at the stunning blonde on the sidewalk, Jenny ran her car smack into the lamp post.
At the age of sixteen, I got a computer from my parents.
After reading the whole book carefully, I did not find the argument convincing.
2) Put the agent into the modifying phrase:
While Jenny stared at the stunning blonde on the sidewalk, her car ran smack into the lamp post.
When I was sixteen, my parents got me a computer.
Even after I read the whole book carefully, the argument did not convince me.