The simple past-tense form always occurs alone, without a helping verb. The past participle is used with a helping verb to form the perfect tenses and the passive voice.
Choose the past-participle form if the verb in your sentence requires a helping verb; choose the past-tense form if the verb does not require a helping verb.
The past-tense forms saw and sank are required because there is no helping verb.
Because of the helping verbs was and had, the past-participle forms are required: was stolen, had fallen.
Survey of verb tenses
Exercise: Irregular verbs 1
Exercise: Irregular verbs 2
Exercise: Irregular verbs 3
helping verb A verb used before a main verb as part of a verb phrase; a form of have, do, or be or a modal verb (can, will, shall, could, would, should, may, might, must).
perfect tense Verb tenses (present perfect, past perfect, future perfect) that signal that an action was or will be completed at the time of another action: have decided, had borrowed, will have visited.
passive voice A verb is in the passive voice when the subject receives the action of the verb: The butterfly was caught by Joan.