Infinitivus pro participio
Infinitivus pro Participio is a idiosyncratic property of Dutch verb-clusters in perfect tense. Compare (i) and (ii):
(i) * Jan heeft gemoeten kiezen Jan has must_PART choose (ii) Jan heeft moeten kiezen Jan has must_INF choose 'Jan has had to choose'
Instead of the expected participial form gemoeten (i) (cf. Jan heeft gemoeten) the form moeten must be used, which is not allowed in perfectives elsewhere: *hij heeft moeten. The form moeten is taken to be the infinitive. IPP is a characteristic property of Verb Raising constructions in Dutch.