In Dutch, a past participle cannot serve as an auxiliary verb. To make the verb act as an auxiliary verb, we must transform the past participle into an infinitive:
when the past participle _itself_
serves as an auxiliary verb,
it turns into an infinitive
In the examples below, the auxiliary verbs are in italics, the independent verbs in bold. The verbs that undergo a transformation from participle to infinitive are underlined.
Note, how the independent participle turns into an infinitive when it serves as an auxiliary verb to support another (independent) verb.
Annelies _had_ een ijsje gewild. | Annelies _had_ wanted an ice-cream.
Thijmen _had_ een ijsje _willen_ eten
(not: gewild eten). | Thijmen _had_ _wanted_ to have an ice-cream.
The independent participle _gewild_ turns into the infinitive _willen_ when it has to support _eten_.
Sanne _is_ naar Amsterdam gekomen. | Sanne _has_ come to Amsterdam.
Liesbeth _is_ _komen_ lopen
(not: gekomen lopen). | Liesbeth _has come_ walking (lit).
The past participle _gekomen_ becomes the infinitive _komen_.
We _hebben_ een uur _staan_ wachten
(not: gestaan wachten). | We _have been_ waiting for an hour.
Ik _had_ beter _moeten_ weten
(not: gemoeten weten). | I _should have_ known better.
The above only applies to the situation where the past participle serves as an auxiliary verb. Consider the following example:
Amir _zou zijn_ gekomen. | Amir _would have_ come.
Here, the past participle is the independent verb and hence does not turn into an infinitive.