Only verbs have past participles. An is not a verb.
The past participle of the word "pick" is "picked".
The past participle of the word "have" is "had."
The word "it" does not have a past participle as it is a pronoun. It is only verbs that have a past participle.
The past participle of the verb to have is had.
The past participle of the word "do" is "done."
The past participle of the word "Find" is the word "Found."
The past participle of the word graduate is graduated
The word "invited" is the past participle of invite.
The past participle of the word 'fall' is 'fallen'.
The word 'worse' is an adjective so doesn't have a past participle. Only verbs have a past tense/past participle.
The past participle is planned.
The past participle is filled.