first, a warning -- the gmat is not a test of rhetoric, so differences that depend on rhetorical considerations will not be tested.
since the vast majority of differences between essential and nonessential (also known as restrictive and nonrestrictive, respectively) modifiers are rhetorical in nature, this means that you'll not often have to worry about deciding whether a modifier should be essential.
nevertheless, here are some considerations that you should know regarding modifiers:
1) modifiers that start with "which" are always nonessential (set off by a comma).
2) modifiers that start with "that" are always essential (not set off by a comma).
3) COMMA -ING modifiers modify the ENTIRETY of the preceding CLAUSE.
4) -ING modifiers WITHOUT COMMAS modify only the preceding NOUN or NOUN PHRASE.
note #3 and #4 here -- this is the only situation in which i can really see the gmat demanding that you be able to distinguish whether you want a modifier with a comma or a modifier without one.
in particular, note that this still doesn't depend on whether the modifier is rhetorically essential or nonessential -- it's still a question of proper modifier usage, i.e., whether it modifies a noun or modifies an entire clause.
here's an illustration:
* the bike path diverges from the highway heading into the city.
--> in this example, we have a -ING modifier that is NOT preceded by a comma.
therefore, this means that it's only the highway bad heads into the city; in other words, the highway heads into the city, and the bike path diverges from that highway (and thus presumably doesn't lead into the city).
* the bike path diverges from the highway, heading into the city.
--> in this example, we have a COMMA -ING modifier, which (as always) modifies the entire preceding CLAUSE.
therefore, this time, it's the bike path that heads into the city (as it diverges from the highway).
Ron has been teaching various standardized tests for 20 years.
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Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
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Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
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