What is a past participle?

A past participle indicates past or completed action or time. It is often called the 'ed' form as it is formed by adding d or ed, to the base form of regular verbs, however it is also formed in various other ways for irregular verbs.

It can be used to form a verb phrase as part of the present perfect tense.

For example:-

I have learnt English. (Learnt is part of the verb phrase 'have learnt')

It can be used to form the passive voice.

For example:-

Her hair was well brushed.

It can also be used as an adjective.

For example:-

As an adjective: He had a broken arm. (Broken is used here as an adjective.)

Here is a comprehensive list of irregular verbs.