indentifying participles vs verbs

This can be very tricky, especially since the GMAT loves to use past and present participles as modifiers. I'll explain the 2 participles separately.

Present participle - It's relatively simple to distinguish between the verb form and modifier (or noun) forms on present participles. To be a verb form the participle needs a helping verb (to be). Without the helping verb, the participle is a modifier or noun.
Examples of verb form:
I am driving to school.
I am eating a PBJ sandwich.
Examples of noun/modifier form:
Driving to school is fun. [Noun]
Eating a PBJ sandwich, I made a mess on my shirt. [Modifier]

Past participle - This is a bit more difficult because the participle can be a regular verb, complex tense verb (needs helping verb), passive verb (needs helping verb) or modifier.
Examples of verb forms:
I worked all night long. [regular past tense verb]
I had purchased a car before the dealer dropped the price. [1st verb is complex with had helping verb, 2nd verb is regular past tense]
The unfortunate man was misdiagnosed by the sloppy doctor. [passive voice verb]
Examples of modifier form:
Planned for weeks in advance, the party was an enormous success. [participle starts a modifying phrase]
The rusted car was worthless. [participle modifies the noun]

This is tricky stuff. Spend some time getting comfortable with the different forms and pay attention to when the GMAT uses participles as modifiers. Modifiers are tested as a grammatical and meaning issue on the GMAT, so improving your ability to spot and understand modifiers will improve your score.

KW