The past participle shows 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).
For regular verbs, you form the past participle and past tense the same way, usually by adding ‑ed.
For irregular verbs, you form the past participle in some other way; the past participle of drink, for example, is drunk.
You can identify the past participle of a verb by completing this sentence: I have [insert verb here]. If you don't know the past participle for an irregular verb like swim, then check the dictionary.
Dictionaries typically list the past tense first (swam), then the past participle (swum), and finally the present participle (swimming).