Sentence Correction

Can any experts direct me to a clear summary post of the SC modification rules for the GMAT? Thank you

Generally speaking, a modifier is a word or a phrase that describes or alters another noun, adjective, or verb. Some of the common errors that have been observed on the GMAT in the past have been:

Substituting an adjective for an adverb

For example: There is speculation that increasing cold weather may have been responsible for the Anasazi move to other areas.
What is increasing: the weather or the cold? If we mean to say the weather, then the sentence is fine. If we mean to say the cold, then the sentence should read:
There is speculation that increasingly cold weather may have been responsible for the Anasazi move to other areas.

Misplaced modifiers

For example: The hunter hid behind the tree armed with a rifle.
Who was armed with the rifle? This sentence seems to indicate that it was the tree. It would be better to say:
Armed with a rifle, the hunter hid behind the tree.

Generally speaking, adjective modifiers should touch the noun they are modifying. Here's another misplaced modifier:

Although affording little cover, the hunter hid behind the tree.
In this sentence, we seem to indicate that the hunter afforded little cover. One way to fix this would be to attach the modifier to the word tree. However, there is another fix:
Although it afforded little cover, the hunter hid behind the tree.
By converting the modifying phrase into a clause (with a subject and a verb), we have eliminated the error.

Dangling modifiers

In this error, the thing under modification doesn't appear in the sentence. For example:
Using a Doppler ultrasound device, fetal heartbeats can be detected.
Who uses the Doppler ultrasound device? Not the fetal heartbeats.
Using a Doppler ultrasound device, doctors can detect fetal heartbeats.

Squinting modifiers

Like many others of his generation of Native American leaders, living in two worlds, Joseph Brant was born into an Iroquois community and instructed in traditional Iroquois ways, he also received an education from English-speaking teachers.
In this sentence the modifying phrase living in two worlds might modify Joseph Brant (the noun after) or Native American leaders (the noun before).
Like many others of his generation of Native American leaders, Joseph Brant lived in two worlds; born into an Iroquois community and instructed in traditional Iroquois ways, he also received an education from English-speaking teachers.
In this sentence, the modifying phrase clearly refers to only one thing.

Pseudo modifiers

Although we have seen above that phrases modify but clauses do not, some experts consider relative clauses to be a form of a modifier.
About 7,500 years ago, a torrent of water poured through the Bosporus into the Black Sea, which raised the water level 500 feet within a year.
This sentence seems to indicate that the Black Sea raised the water level 500 feet within a year.
About 7,500 years ago, a torrent of water poured through the Bosporus into the Black Sea, raising the water level 500 feet within a year.
In this sentence, the two modifiers are adverbial modifiers. They do not need to touch what they are modifying.

Absolute modifiers

What's wrong with this sentence?

Scientists have observed large concentrations of heavy-metal deposits in the upper twenty centimeters of Baltic Sea sediments, which are consistent with the growth of industrial activity there.
It appears to say that Baltic Sea sediments are consistent with the growth of industrial activity in the area. A better sentence would be:
Scientists have observed large concentrations of heavy-metal deposits in the upper twenty centimeters of sediments from the Baltic Sea, findings consistent with the growth of industrial activity in the area.
In this sentence, we see an absolute modifier. It occurs when you have a complete sentence, a comma, a new noun, and a modifying phrase attached to that noun. For example:
John went to the party, a decision that he would later regret.

Noun Phrases

Architects and stonemasons, huge palace and temple clusters were built by the Maya without the benefit of the wheel or animal transport.
Who or what were architects and stonemasons? Surely not the palace and temple clusters.
Architects and stonemasons, the Maya built huge palace and temple clusters without the benefit of animal transport or the wheel.
In the second sentence, the modifier touches what it is meant to modify.

Prepositional Phrases

Local residents claim that San Antonio, Texas, has more good Mexican American restaurants than any other city does in the United States.
In this sentence, the phrase "in the United States" touches the verb. It seems to indicate that Mexican American restaurants do things in the United States.
Local residents claim that San Antonio, Texas, has more good Mexican American restaurants than does any other city in the United States.
In this sentence, the phrase touches the word city, clarifying the modifier.

Exceptions to the Touch Rule

Exceptions to the touch rule always seem to involve pseudo modifiers such as relative clauses. For example:
Emily Dickinson's letters to Susan Huntington Dickinson, which were written over a period beginning a few years before Susan's marriage to Emily's brother and ending shortly before Emily's death in 1886, outnumber her letters to anyone else.
In this sentence, some people may think that the comma which is ambiguous as the noun it touches (Susan Huntington Dickinson) was not written over a period. In reality, this sentence is clear and unambiguous because which always refers to an object-never to a person.