A past participle is the form of a verb used with have (have, has, had) and the verb to be . Usually it has the same -ed ending as the past tense of a verb.
I have tried to be clear.
 
But sometimes the verb is irregular, such as understand, which has the past tense and past participle understood.
I have understood all of that.
 
Just to make things easy, the past participle can be different from the irregular past tense. For example, the past tense of begin is began, but the past participle is begun.
I have begun to understand.
 
Irregular past participles often end with -en and are used in past perfect, present perfect and future perfect tenses, as well as the passive voice.
I have been chosen to explain this to you. And here it is spoken and written for you to make sure that it is not forgotten.