Active voice

  is

Auxiliary

  used

Verb

  in

Adposition

  a

Determiner

  clause

Noun

  whose

Pronoun

  subject

Noun

  expresses

Verb

  the

Determiner

  main

Adjective

 verb's  agent

Noun

.

Active voice is used in a clause whose subject expresses the main verb's agent.

Active voice is used in a clause whose subject expresses the main verb's agent.