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
Exercises:
Irregular verbs 1
Irregular verbs 2
Irregular verbs 3