Regular verbs typically form their past participles by adding ‑ed. Irregular verbs, however, form their past participles in some other way. For example, they change an internal vowel (I drink, I drank, I have drunk), or they add “-en” to form the past participle (I choose, I chose, I have chosen).
Past participles show up in verb conjugation in two ways: (1) to form the perfect tenses with the auxiliary verb have (I have decided to retire) and (2) to form the passive voice with the auxiliary verb to be (The case was decided by the court).
But past participles also form past-participial phrases. These phrases always act as adjectives (the case decided by the court). You can start sentences with a past-participial phrase. Just make sure it modifies the grammatical subject of the sentence. Here are some examples of past-participial phrases:
Written by the personnel director, the office manual details the rules of employment. This letter, mailed on January 17th, demanded a response. See infinitive phrase and present-participial phrase.