Page 533 - Practical English Usage 3ed - Michael Swan, Oxford

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sentence structure (4): information structure different ways of organising information
When we talk about a situation, we can usually organise the information in various ways - for example, by choosing different elements of the situation as the subject of a clause or sentence.
The storm blew Margaret's roof off. Margaret's roof was blown off in the storm. Margaret had her roof blown off in the storm.
The way we choose to organise information in a clause or sentence can depend on what has been said before, on what the listener already knows, or on what we want to emphasise. This is a complicated area of English grammar. Some guidelines are given below.
normal order: important new information last
Most often, a clause or sentence moves from 'known' to 'new': from low to high information value. So we often choose as the subject a person or thing that is already being talked about or that has already been mentioned, or something that the speaker and hearer are both familiar with, or even some new information that is not the main point of the message. The important new information generally comes at the end of a clause or sentence.
How's Joe these days? - Oh, fine. He's just got married to a very nice girl. (More natural than ... A very nice girl's just got married to him.)
My father was bitten by a dog last week.
(More natural than A dog bit my father last week.)
Our dog bit the postman this morning.
(More natural than The postman was bitten by our dog this morning.)
I can't find my clothes. - Well, your trousers are under my coat. (More natural than ... My coat's on your trousers.)
To avoid beginning a clause with a completely new element, we can use the there is structure. For details, see 587.
There's a cat on the roof. (More natural than A eat's on the roof.) For 'known' and 'new' infonnation with as, sillce and because, see 72.
getting the right SUbject: actives, passives, etc
In many situations, there is an 'agent' (the person or thing who does something) and a 'patient' (the person or thing that something is done to). If we want to make the agent the subject, we can usually do this by choosing an active verb form (see 10).
The storm blew Margaret's roof off.
Somebody's dropped ketchup all over the floor.
If we want to make the patient the subject, we can usually do this by choosing a passive verb form (see 412).
Margaret's roof was blown off in the storm.
Ketchup has been dropped all over the floor.
Ifwe want to make something else the subject, we can often do this by using a structure with have + object + past participle (see 238.3).
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sentence structure (4): information structure 512
Margaret had her roof blown off in the storm. The floor has had ketchup dropped all over it.
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