Page 324 - Practical English Usage 3ed - Michael Swan, Oxford

kinds of English (3): spoken and written English 310
If enough people follow the trend, this will sooner or later become the normal meaning, and the usage will have become correct.
310 kinds of English (3): spoken and written English 1 length and complexity; organisation of sentences
In writing, sentences can be planned in advance and revised, so there is time to build up complex structures. Spoken structures are usually simpler. Subjects, in particular, tend to be very short in speech. A typical written sentence:
The group ofyoung people who were sitting at the next table were making so much noise that my friends and I found it difficult to continue our conversation.
In speech, we might say something more like:
There were a lot ofyoung people at the next table. They were making so much noise we couldn't talk.
Written language is mostly made up of complete sentences. In conversation, complete sentences are often unnecessary.
Whenareyouseeingher? - Halfpasteight. - Atyourplace? - No, atAndy's. Spoken sentences are often more loosely organised than written sentences, and the information may be 'spaced out' more by putting some of it before or after the main sentence (see 514).
Last Wednesday it was, I was just going to work, ...
This guy who rang up, he's an architect. Well, he said . .. They work very hard, most of them.
'Fronting' - putting something other than the subject at the beginning (see 513) - is more common in speech than in writing.
People like that I just can't stand. Strange people they are!
2 structures
Some structures - for example, relative clauses with whom - are most common in a formal style. Since speech is more often informal, and writing is more often formal, these structures are most common in written English. Other structures - for example, contractions like he's, can't - are typically informal, and are most common in speech. (For more about formal and informal language, see 311.) Some structures are common in speech, but hardly ever found in writing - for example, declarative questions (see 481):
You live with your parents?
certain conditional structures (see 262):
It would be good if we'd get some rain. certain relative structures (see 498.16):
It's ridiculous to sing songs that you don't know what they mean.
and some kinds of ellipsis (see 179):
Couldn't understand a word.
Progressive and past verb forms are often used in speech in order to sound less definite or direct (see 436).
I was hoping you could lend me some money.
And structures whose purpose is to keep a conversation going (e.g. reply questions - see 484) are naturally only used in speech.
We had a lovely holiday. - Did you?
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