Participial phrases begin with a present participle (verb ending in -ing) or a past participle (verb often ending in -ed or -en). They function as adjectives to provide more information about a noun or pronoun.
Example: "Running down the street, he saw an old friend." (The participial phrase "Running down the street" describes "he.")
Infinitive phrases begin with "to" followed by the base form of a verb. They can function as nouns, adjectives, or adverbs in a sentence.
Example: "He wants to run a marathon." (The infinitive phrase "to run a marathon" acts as the object of the verb "wants.")