Page 420 - Practical English Usage 3ed - Michael Swan, Oxford

passives (3): When do we use passive structures? 414
2 putting the news at the end
We often prefer to begin a sentence with something that is already known, or that we are already talking about, and to put the 'news' at the end. This is another common reason for choosing passive structures. Compare:
fohn's painting my portrait. (active verb so that the 'news' - the portrait- can go at the end)
Nice picture. '" Yes, it was painted by my grandmother. (passive verb so that the 'news' - the painter - can go at the end)
3 keeping the same subject
In order to keep talking about the same person or thing, it may be necessary to switch from active to passive and back.
Hewaitedfortwo hours;thenhewasseenbyadoctor;thenhewassentback to the waiting room. He sat there for another two hours - by this time he was getting angry. Then he was taken upstairs and examined by a specialist, after which he had to wait for another hour before he was allowed to go home. (More natural than He waited for two hours; then a doctor saw
him .. .)
4 putting heavier expressions at the end
Longer and heavier expressions often go at the end of a clause, and this can also be a reason for choosing a passive structure.
I was annoyed by Mary wanting to tell everybody what to do.
(More natural than Mary wanting to tell everybody what to do annoyed me - the phrase Mary . .. dn would make a very long subject.)
5 meaning and grammar
Meaning and grammar do not always go together. Not all active verbs have 'active' meanings; for instance, if you say that somebody receives something or SUffers, you are really saying that something is done to him/her. Some English active verbs might be translated by passive or reflexive verbs in certain other languages: e.g. My shoes are wearing out; She is sitting; Suddenly the door opened. And some English passives might be translated by active or reflexive verbs: e.g. I was born in 1956; English is spoken here.
Some verbs can be used in both active and passive forms with similar meanings: for example to worry / to be worried; to drown / to be drowned (see 165). Sometimes active and passive infinitives can be used with very similar meanings: for example There's a lot ofwork to dn / to be done (for details, see 287).
For more about verbs lilc:e open, see 609.
For more about reflexive verbs, see 493.
For active and passive past participles, see 409.
For -ing forms with passive meanings after need and want (e.g. My watch needs cleaningJ,
see 296.3.
For more about the way information is organised in sentences, see 512.
page 388