Present participles, verbals ending in -ing, and past participles, verbals that end in -ed (for regular verbs) or other forms (for irregular verbs), are combined with complements and modifiers and become part of important phrasal structures. Participial phrases always act as adjectives. When they begin a sentence, they are often set off by a comma (as an introductory modifier); otherwise, participial phrases will be set off by commas if they are parenthetical elements.
The stone steps, having been worn down by generations of students, needed to be replaced. [modifies "steps"]
Working around the clock, the firefighters finally put out the last of the California brush fires. [modifies "firefighters"]
The pond, frozen over since early December, is now safe for ice-skating. [modifies "pond"]