A past participle is a nonfinite verb form that can be used as an adjective, to form perfect verb tenses, and to form the passive voice. It is one of two types of participles, along with present participles. The past participle of regular verbs is usually formed by adding the suffix "-ed" (e.g., "learn" becomes "learned"), while the past participle of irregular verbs has numerous endings like "-en," "-n," "-ne," and "-t" (e.g., "kneel" becomes "knelt") . The past participle is used in several tenses, especially perfect forms, and is generally used with an auxiliary verb like "has," "have," or "had" to express the perfect aspect. It can also be used as an adjective to modify a noun or pronoun, and in formal written English, it can be used in participle phrases. In passive voice sentences, the past participle is used to describe the action that was done to the subject of the sentence.