Double Infinitive: When modals are used in the present perfect tense with a dependent infinitive, the past participle is not used. The infinitive of the modal acts as the past participle. Logically, I had to go home would be translated as ich habe nach Hause gehen gemußt. However, it is actually Ich habe nach Hause gehen müssen. When there is no other infinitive in the sentence, then the past participles of the modals are used. I had to would be translated as Ich habe gemußt.