Grammar: Complementary Infinitive
Very often infinitives are used to complete ("complement") the meaning of another verb. In these situations, the subject of the infinitive is often the same as the subject of the main verb. Why is that important? If the subject of the main verb is in the nominative case, and if the infinitive contains a participle, then that participle agrees in gender, number and case (nominative) with the subject of the main verb.