A past participle, by itself, cannot be the main verb of a sentence. When a past participle is combined with another verb, called a helping verb, however, it can be used to make the present perfect tense and the past perfect tense.

The present perfect tense is used for an action that began in the past and either continues into the present or was completed at some unknown time in the past.

My car has stalled several times recently.

Use had plus the past participle to make the past perfect tense. The past perfect tense is used for an action that began in the past and ended before some other past action.

My car had stalled several times before I called the mechanic.

A sentence that is written in the passive voice has a subject that does not perform an action. Instead, the subject is acted upon. To create the passive voice, combine a form of the verb be with a past participle.

Most sentences should be written in the active voice, which means that the subject performs the action.