The simple past tense is used to describe actions or events that occurred and were completed at a specific point in the past. It often indicates a single, completed action without any connection to the present. For example: "She walked to the store yesterday." The past perfect tense is used to describe an action that occurred before another action in the past. It emphasizes the completion of one action before another took place. It is formed by using the auxiliary verb "had" followed by the past participle of the main verb. For example: "By the time they arrived, I had already finished my dinner." In this sentence, the past perfect "had finished" indicates that the action of finishing dinner occurred before the action of arriving.