PASSIVE VOICE

La voz activa se emplea muy frecuentemente en nuestro idioma, pero en inglés puede haber muchos motivos para preferir la voz pasiva:

1- El sujeto es obvio: The thief was arrested (by the police)

2- Para centrar la atención en el complemento : That beautiful song was sang by....

3- En estilo formal: The article will be finished next week.

4- En artículos científicos: Radium and polonium were discovered by Marie Curie.

En primer lugar, en la voz activa, el WORD ORDER correcto es sujeto+verbo+complementos:
                              I bought flowers to Mary

En la voz pasiva, el WORD ORDER correcto es:

C.D. ó C.I. de la voz activa como nuevo sujeto de la pasiva:
                              Flowers ...
VERBO: to be (en el tiempo del verbo principal de la voz activa, y conjugando con el nuevo sujeto) + el verbo principal SIEMPRE en participio (3ªcolumna de los verbos irregulares, o la terminación -ed en los verbos regulares)
                              ...were bought to Mary
COMPLEMENTO AGENTE: Es el sujeto de la voz activa:
                              ...by me.

RESULTADO:
Activa: I bought flowers to Mary
Pasiva: Flowers were bought to Mary by me.

Las oraciones activas que tienen C.D. y C.I., pueden hacer 2 pasivas:
Pasiva de C.D.:
                                 Flowers were bought to Mary by me.
Pasiva de C.I. :
                                 Mary was bought some flowers by me.

Tense	Active	Passive
present simple	I write a letter.	A letter is written (by me).
present continuous	I am writing a letter.	A letter is being written (by me).
past simple	I wrote a letter.	A letter was written  (by me).
past continuous	I was writing a letter.	A letter was being written (by me).
present perfect	I have written a letter.	A letter has been written (by me).
past perfect	I had written a letter.	A letter had been written (by me).
future simple	I will write a letter.	A letter will be written (by me).
future perfect	I will have written a letter.	A letter will have been written(by me).

Ejercicios

1- PASSIVE WITH SIMPLE PRESENT

http://www.ego4u.com/en/cram-up/grammar/passive/exercises

http://www.ego4u.com/en/cram-up/grammar/passive/exercises?simple-present
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?simple-present-2

2 - PASSIVE WITH PRESENT CONTINUOUS
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?present-progressive

3 - PASSIVE WITH SIMPLE PAST

http://www.ego4u.com/en/cram-up/grammar/passive/exercises?02

http://www.ego4u.com/en/cram-up/grammar/passive/exercises?simple-past
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?simple-past-2

4 -PASSIVE WITH PAST PROGRESSIVE
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?past-progressive

5 - PASSIVE WITH PRESENT PERFECT

http://www.ego4u.com/en/cram-up/grammar//exercises?03

http://www.ego4u.com/en/cram-up/grammar/passive/exercises?present-perfect
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?present-perfect-2

6 - PASSIVE WITH PAST PERFECT
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?past-perfect

7 - PASSIVE WITH FUTURE

http://www.ego4u.com/en/cram-up/grammar/passive/exercises?04

http://www.ego4u.com/en/cram-up/grammar/passive/exercises?future-1
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?future-will-2
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?future-2

8 - PASSIVE WITH CONDITIONAL
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?conditional-1
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?conditional-2

9 - PASSIVE WITH AUXILIARY VERBS
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?05

10 - PASSIVE WITH 2 OBJECTS (I.O.)
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?07

11 - PASSIVE WITH 2 OBJECTS (D.O.)
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?08

12 - PEOPLE SAY.....   IT'S SAID..... (IMPERSONAL PASSIVE)
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?09
http://www.englishexercises.org/makeagame/viewgame.asp?id=6854

13 - TEXTS
a -http://www.ego4u.com/en/cram-up/grammar/passive/exercises?text01
b -http://www.ego4u.com/en/cram-up/grammar/passive/exercises?text02
c - Part 1. http://www.ego4u.com/en/cram-up/writing/summary/exercises
   - Part 2. http://www.ego4u.com/en/cram-up/writing/summary/exercises?ex02
   - Part 3. http://www.ego4u.com/en/cram-up/writing/summary/exercises?ex03

14 - MIXED EXERCISES
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?06
http://www.ego4u.com/en/cram-up/tests/hadrians-wall