The past simple is a tense used to indicate an action that took place and was completed in the past. It is formed by adding -ed to regular verbs and using the past form of irregular verbs. We use it for actions that happened at a specific time in the past or for a series of completed actions. For example, "She walked to school yesterday." To form the past simple, use the base form of the verb for most subjects, but add -s or -es to the base form for third person singular subjects.