Participles are "verbals," or forms of verbs, functioning as adjectives. Gerunds are verbals functioning as nouns. This sounds simple enough, but many people find them tricky to recognize and use correctly -- perhaps because they remember being intimidated and confused by these concepts in school. The good news is that both participles and gerunds are simpler than people realize, and a few simple tricks can make spotting them and using them properly much easier.
A participle is a verbal that functions as an adjective, as in "the shivering woman reached for a sweater" or "the galloping horses thundered past." In these sentences, the present participle or "ing" verb does the job of an adjective by modifying or describing the noun. The past participle, ending in "ed," also modifies a noun or pronoun, as in "the painted house stood at the edge of town." Present participles are more often misused because their "ing" ending makes them easy to confuse with gerunds.
A gerund is a verbal ending in "ing" that functions as a noun. In the sentence "Swimming is my favorite sport," the "ing" verbal is the subject. In the sentence "I love reading," the "ing" verbal is the direct object of the sentence, again doing the job of a noun.
Common Problems with Participles and Gerunds
A dangling participle is a participial phrase, usually though not necessarily at the beginning of a sentence, that is not attached to the noun it's supposed to modify. For example, "Tripping on the stairs, the tea set slipped from my hands." Who tripped on the stairs: the tea set or the speaker? Such "dangling" participles can be avoided by making sure that whatever comes immediately after the participial phrase is the noun being modified. If I tripped on the stairs, the only thing that can come after "tripping down the stairs" is "I."
The most common problem in gerund use is failure to use the possessive pronoun in a sentence that includes a gerund. For example, "I really hate your always coming late to meetings" is correct. The direct object of "I hate" -- the thing I am hating -- is the gerund "coming late to meetings." It just happens to be your coming late to meetings in this case. An easy mistake is to say instead, "I really hate you always coming late to meetings." This is incorrect because it's not you I hate, it's your tardiness. The possessive pronoun is a reminder that the gerund is the object and the "main point" of the action.
Telling Participles and Gerunds Apart
The simplest way to tell participles and gerunds apart is to determine whether an "ing" word or phrase could be removed while leaving a complete sentence. If a participle is removed from a sentence, what is left behind will still be a complete sentence with a subject and a verb. For example, in the sentence "Sipping lemonade, we sat on the porch swing," the phrase "sipping lemonade" could be discarded; the sentence "We sat on the porch swing" is perfectly correct. (As in this example, a comma often hints at a participial phrase.)
On the other hand, if a gerund is removed from a sentence, what remains will no longer be a complete sentence. In the sentence "Sipping lemonade is my favorite way to spend a summer afternoon," the phrase "sipping lemonade" cannot be removed. Without it, what remains is a sentence fragment with no subject. Similarly, if the "ing" gerund phrase is removed from the sentence "I enjoy dancing," what remains is, again, a sentence fragment that is missing key information. By mentally removing the "ing" word or phrase to see if a complete sentence remains, you can usually determine quickly whether you have a participle or a gerund.