Gerunds and participles often look similar; they are both verb forms that end in -ing, but serve different functions in English grammar. A gerund serves as a noun while a participle serves as an adjective. And because the same word can often be either a participle or a gerund, to identify a word's role, you must examine how it is being used.
Look for the Subject
Generally, if a possible gerund can be shown not to act as a modifier, then you can deduce that it is functioning as a noun and therefore is a gerund. More specifically, if the possible gerund functions as the entire subject of the sentence and does not take any additional punctuation, it is a gerund. For example, in the sentence, "Setting the table is necessary before dinner," "Setting the table" is the subject and takes no punctuation; "setting" is a gerund.
Look for Participial Phrases
Because gerunds often serve as subjects, they are often confused with participial phrases at the beginnings of sentences. Two defining qualities of a participial phrase are the presence of punctuation that sets it apart from the rest of the sentence, and the phrase itself, which modifies the subject instead of being the subject. In the sentence, "Unsheathing his sword, the knight stormed the castle," "Unsheathing his sword" is a participial phrase because it is set off by a comma and modifies the subject, "the knight."
Is It an Adjective?
Sometimes participles function as adjectives, and so another way to tell that a word is a participle and not a gerund is to identify it as a modifier, which often precedes the noun it modifies. Consider the following example: "She offers stinging criticisms." Here, the word "stinging" serves as an adjective by modifying "criticisms," and in no way functions as a noun; it is clearly a participle.
Is It Being Modified by an Adverb?
Sometimes a word in adverbial form will be attached to a potential participle/gerund, signifying its use as an adjective, and is therefore a participle and not a gerund. In the sentence: "Quietly muttering to himself, John struggled through the test," the adverb "quietly" modifies "muttering," signalling the identity of "muttering" as a participle.
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