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