Como ya hemos visto en el 8º capítulo 8.3.2 algunos (alrededor de doscientos) participios perfectos son irregulares.

Para repetir otra vez los más importantes

infinitivo simple past perfect participle
to be was, were been
to go went gone
to buy bought bought
to come came come
to give gave given
to see saw seen
to eat ate eaten
to write wrote written
to read read read
to know knew known
ejemplos
He had gone to bed. Se había ido a la cama.
We had bought a new car. Habíamos comprado un nuevo coche.
They had seen you naked. Te habían visto desnudo.
She had eaten lots of apples. Habían comido muchas manzanas.
I had known it. Lo había sabido.