Modifying phrases can help describe something more precisely or explain something more fully.
If modifiers are not clearly linked to the noun they are modifying, these phrases can also make a sentence confusing. For example:
With its contents spilling onto the highway, Lieutenant Stanley ran toward the overturned tanker truck.
The way this is written, it seems as though the contents of Lieutenant Stanley are spilling; it is actually the tanker truck’s contents. To make this sentence clear and grammatically correct, it should be rewritten like this:
Lieutenant Stanley ran toward the overturned tanker truck, which was spilling its contents onto the highway.
A modifying phrase or clause should clearly refer to a particular word in the sentence. A modifying phrase or clause that does not sensibly refer to any word in the sentence is called a dangling modifier. The most common sort of dangler is an introductory modifying phrase that’s followed by a word it can’t logically refer to.
Wrong: Desiring to free his readers from superstition, the theories of Epicurus are expounded in Lucretius’s poem De rerum natura.
The problem with this sentence is that the phrase that begins the sentence seems to modify the noun following it: theories. In fact, there is really nowhere the modifier can be put to make it work properly and no noun to which it can reasonably refer (Lucretius’s, the possessive, is functioning as an adjective modifying poem). Get rid of dangling constructions by clarifying the modification relationship or by making the dangler into a subordinate clause.
Correct: Desiring to free his readers from superstition, Lucretius expounded the theories of Epicurus in his poem De rerum natura.
Now the phrase desiring to free his readers from superstition clearly refers to the proper noun Lucretius.