They may be used to affect an informal, friendlier tone that is easier on the eye and mind, but in formal writing they are discouraged or not used at all.
Shortening and combining words – don’t (do + not), can’t (can + not), I’ve (I + have) – is like a syntactical facelift: by pulling words together, it can tighten up a sagging sentence and put a smile between its cheeks.
However, contractions are mostly confined to direct speech (i.e., within quotation marks) and rarely used in reported speech (i.e., a summary of what someone actually said), and most formal style guides ban them outright
They are never acceptable in academic manuscripts, for example, except when directly quoting another source. The gold standard of scientific style, the American Chemical Society (ACS) Style Guide, prohibits writers from using them. News organisations such as the Associated Press urge writers to eschew their use. Many copywriters – and schoolteachers – believe strongly that they are sloppy.
There is no doubt that contractions are more frequent in speech than in writing, and are, in turn, more frequent in informal than formal contexts.
But even in crisp, precise settings, it is simply not always possible to replace ‘don’t’ or ‘can’t’ with ‘do not’ or ‘cannot’. After all, try contracting ‘Why can’t I?’ (‘Why cannot I?’, ‘Why can I not?’) and you can see the problem.
Meaning is a garden with many shades: there are subtle differences in emphasis between the use of ‘Do not walk on the grass’ and ‘Don’t walk on the grass’ that will be conveyed only by context and overall tone of voice. In the former, one gets the sense that walking on the grass is prohibited; in the latter, that it is disapproved of.
The writer’s choice will be informed by the whole range of nuanced factors that together comprise tone – the overall context and theme of a body of writing, a brand’s image, the creative director’s brief and instinct etc.
While the position that we take on the use of contractions may be very personal, based upon our background and experience, at the end of the day the words that we select have a job to do.
If their aim is to achieve a flowing, colloquial tone that engages the customer as if they were in a conversation … then don’t say can’t.