ASK DOCTOR DOROTHY PASTENSE FULLSTOP:
When should - or shouldn't - we use contractions in writing?
There's one simple answer to this - it's all a matter of style. Okay, let's make it two - register as well.
Contractions are largely used in informal and spoken English. They are often used when reading out loud, for example, the speaker reads something as He has come back as he's come back, no matter if the text is written as He has as he's. But the reader may have to be aware. Look at these two examples:
Fred entered the room. 'Where's Jane?'
'Jane's gone to the shop,' grinned Brian.
'Do you know when she's back?' asked Fred casually.
Fred entered the room, looking furious. 'Where's Jane?'
'Jane's gone to the shop,' said Brian.
'She said she'd be back.'
'And do you know when she is back?' asked Fred angrily.
What do you notice? In the last line of dialogue, Fred deliberately avoids a contraction (And do you know when she is back?) because he wants to stress his unhappiness. It's now a formal situation, he wants Jane in the office, and he wants to know precisely when she will be in the office as he feels time is being wasted. In the first example, all is calm, relaxed, and is an informal situation.
So onto texts. When studying the four registers of English (conversation, fiction, news and academic), from my copy of the Longman Student Grammar (Biber, Conrad, Leech, 2002), we see that roughly 70 per cent of available contractions with verbs are used in conversation. (We can assume this includes informal emailing.) In fiction, this drops to around 45 per cent (with a significant drop with the verb 'have' as in I've, you've, she's, etc). News barely reaches 10 per cent while academic writing barely registers at all.
The use of contractions in writing also depends on various factors such as newspaper style guides (tabloids are more likely to use them, than say, serious newspapers, or broadsheets), the likely audience of the book or magazine, and to who (or whom) you are writing to: a well-written job application, for example, would not recommend the use of contractions.
But to say that all formal writing outside letters, email and literature should not contain contractions would be incorrect. My copy of Questions of English, (Oxford: Marshall, McDonald, 1995) has, for example, used I'm and wasn't (page 98) and don't (page 99), but their use really does not look out of place. There are two reasons for this: First, they are being used with pronouns (I, you, he, she, it, we, they), and secondly, the book is aimed at everyday users of English who ask frequently asked questions, and the responses often have to be entertaining, even with a touch of informality.
Now I have to say that TEE has largely dropped the contraction in its official responses, but keen-eyed readers would have seen that some did get in there - clearly someone did not shut the contraction door properly. Let's appears on page 141, I'm and that's are in the acknowledgements on page 5. But they do not look out of place in a book that takes the same register as the Questions of English book, and I am not going to worry about them too much, so they and perhaps a few more might appear in the second edition. Well, they appear in the first line of this explanation!
So in summary - contractions are fine for informal use. No contractions - formal. But it really depends on who your audience is.