The passive voice

Utilizamos a voz passiva com o intuito de fazer com que a ação fique mais evidente do que quem a realiza ou pela ausência de informações a respeito de quem cometeu tal ato ou, simplesmente, pelo fato de a publicação preferir evidenciá-la em detrimento de quem a realizou.

Quando o foco está em quem realiza a ação usamos a voz ativa (active voice):

Mr Smith locks the gate at 6 o’clock every night.

The storm destroyed dozen of trees.

Quando o foco está em quem sofre a ação, usamos a voz passiva (passive voice):

The gate is locked at 6 o’clock every night.

Dozens of trees were destroyed.

A voz passiva é formada pelo auxiliar be e o past participle do verbo principal:

Two new stores were opened this year.

The room had been cleaned.

Se você usar a voz passiva e você quiser mencionar quem realizou a ação, use by:

He had been poisoned by his girlfriend.

She was brought up by an aunt.

Veja outros exemplos:

Simple Present –  The gate is locked at 5 pm.

Present Progressive – Jobs are still being lost.

Simple Past – He was killed in a plane crash.

Future – The books will be sent to you.

Present Perfect – The car has been stolen.

Past Perfect – This phone bill had not been paid.

Future Perfect – Further information will have been sent by then.

Modals – What can be done?

Referência:
Student’s Grammar. Harper Collins Publishers.