Hören can be used in a so-called "accusative with infinitive" construction (as in English): Ich hörte ihn rufen. – “I heard him call.” If such a sentence is in the perfect or pluperfect tense, the infinitive usually replaces the past participle: Ich hatte ihn rufen hören. – “I had heard him call.” The use of the past participle instead does occur in some speakers, but is ungrammatical to many others.