build it

HAD + PAST PARTICIPLE
(simple past of be + pa.pa.)

I had finished lunch when he visited me
You had answered before I even asked you..
She had had cleared the table when I came in.
They left the building as soon as they had heard the fire alarm.

past participle of regular verbs

past participle = infinitive + ED

talked
waited
asked
answered

special: drop the end -E of the infinitive

invite -> invited
receive -> received

past participle of irregular verbs

Irregular means that they don't follow rules.
And so we can't describe the form in general.
There is only one way to know the correct form: memorize it.

Some examples:
to break -> broken
to drink -> drunk
to eat -> eaten
to grow -> grown
to sit -> sat

Here is a list of irregular verbs and their forms.
If you click the link, you can see the list.
If you right click it, you can download the list to your computer.