When to use past participle? And diff.between past and past participel

Dear Student, 

Past participle is a form of verb which is used in the present perfect and past perfect tense. They cannot be used on their own. Auxiliary verbs like has/ have or had is used with past participles.

Eg: She had finished her breakfast by 8 o' clock yesterday. ( finished is the past participle here)

Difference between past and past participle

This part of the question is not clear. I am assuming you want the difference between the past and past participle form of verb.
Accordingly, here is the answer :

The past form of the verb can be used independently. The past participle form of the verb is always used with an auxiliary verb.
Eg: The verb sing. 
       Past form - sang
       She sang the song.
       Past participle: sung
       She had sung the song when you called.

Regards,