Note: To read about “which vs. that” on the GMAT®, see this post. Here, I focus on the various uses of which.
To begin, which is used after a comma in order to set off a non-essential (aka a non-restrictive) clause — that is, a clause that can be removed from a sentence without disturbing its basic grammatical structure.
This type of clause can be inserted in the middle of a sentence, between two commas:
Correct: A group of birds known as enantiornithes, which went extinct along with dinosaurs at the end of the Cretaceous period, dwelled primarily in the Hukawng Valley in northern Myanmar.
It can also be inserted at the end of a sentence, also after a comma:
Correct: Mined in the Hukawng Valley in northern Myanmar, Burmese amber deposits contain what could be the largest variety of animal and plant life from the Cretaceous period, which lasted from 145.5 to 65.5 million years ago.
When the “which” clauses are removed from both of these sentences, the sentences still make grammatical sense.
Correct: A group of birds known as enantiornithes […] dwelled primarily in the Hukawng Valley in northern Myanmar.
Correct: Mined in the Hukawng Valley in northern Myanmar, Burmese amber deposits contain what could be the largest variety of animal and plant life from the Cretaceous period.
Now, as a general rule, which should refer to the noun that immediately precedes it (the antecedent or referent). This is sometimes referred to as the “which touch” rule.
Why must which follow the noun to which it refers? Because clauses begun by which are modifiers (descriptive phrases), and for the sake of logic, modifiers must be placed next to the nouns or phrases they modify.
For example, take another look at this sentence:
Correct: Mined in the Hukawng Valley in northern Myanmar, Burmese amber deposits contain what could be the largest variety of animal and plant life from the Cretaceous period, which lasted from 145.5 to 65.5 million years ago.
Here, which lasted from 145.5 to 65.5 million years ago logically refers to the Cretaceous period.
If which does not logically refer to the noun that precedes it, but rather to another noun elsewhere in the sentence, a misplaced modifier is created.
Incorrect: Mined in the Hukawng Valley in northern Myanmar, Burmese amber deposits from the Cretaceous period contain what could be the largest variety of animal and plant life, which lasted from 145.5 to 65.5 million years ago.
Logically, the largest variety of plant and animal life did not last from 145.5 to 65.5 million years ago — but that is what the sentence literally implies. To fix the sentence, it is necessary to place the Cretaceous period next to the clause begun by which.
In everyday writing, which is also commonly used to refer to nouns that are not explicitly mentioned in sentences. Essentially, which word is used to refer to entire ideas — the noun is merely implied. On the GMAT, this usage is incorrect.
Incorrect: The presence of flight feathers on a recently discovered fossil of an infant bird is reinforcing the idea that some ancient bird species could fly when they hatched, which made them less dependent on parental care than most modern birds are.
In this case, which is not preceded by a noun. Although the intended meaning of the sentence is clear enough, it is not acceptable by GMAT standards because which technically refers to some ancient bird species’ ability to fly when they hatched, but that noun does not actually appear. In order to correct the sentence, it is necessary to supply the noun.
Correct: The presence of flight feathers on a recently discovered fossil of an infant bird is reinforcing the idea that some ancient bird species could fly when they hatched, an ability that made them less dependent on parental care than most modern birds are.
Another frequent correction involves placing a participle (-ING form of a verb) after the comma:
Correct: The presence of flight feathers on a recently discovered fossil of an infant bird is reinforcing the idea that some ancient bird species could fly when they hatched, making them less dependent on parental care than most modern birds are.
Note that this is an extremely common error on the GMAT, appearing in numerous wrong answers in the Official Guide.
Now, there is one situation in which things become a bit more ambiguous. It involves noun phrases — typically, a noun followed by a prepositional phrase. In such cases, it is sometimes acceptable for the noun referred to by which not to appear immediately before which.
Correct: Among the earliest complex civilizations in the Americas was the Olmec culture of Mexico, which flourished on the Gulf coast beginning in around 1500 B.C.E.
If you were to consider only at the noun immediately before which, this sentence would not really make sense. Presumably the Olmec culture, not Mexico, flourished on the Gulf coast beginning in around 1500 B.C.E. But that interpretation of the “which” rule is a bit too literal.
In fact, the intended meaning of the sentence is clear. The noun Mexico is part of the prepositional phrase that makes up the complete referent: the Olmec culture of Mexico. As a result, it is acceptable for a noun other than the technical referent to precede which.
Ultimately, these constructions must be considered on a case-by-case basis. The GMAC is more interested in your ability to identify answers that make the most sense than it is in always forcing you to apply rules in the most literal way possible. Some questions will sit on the border between grammar and style, and you must develop a tolerance for constructions that are not 100% clear cut. That can be a challenge, particularly if you are not a native English speaker; however, if you take the time to stop and think about what’s actually being said, there’s a reasonable chance you can find your way to the answer.