This is “Forming Participles”, section 1.10 (from appendix 1) from the book Writers' Handbook (v. 1.0).
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Participles are verb forms that combine with auxiliary verbs to create different tenses.
To form perfect tenses, use had, has, or have with the past participle.
Example: My dog has eaten twice today.
To form progressive tenses, use a form of the verb to be with the present participle, or gerund.
Example: My dog is eating a treat.
To write in passive voice, use a form of the verb to be with the past participle.
Example: The treat was eaten by my dog.
See Chapter 15 "Sentence Building", Section 15.2 "Choosing Appropriate Verb Tenses" for a more extensive overview of the relationships between participles and verb tenses, and see Chapter 16 "Sentence Style", Section 16.3 "Using Subordination and Coordination" for more on passive voice constructions.