I assume you understand, by now, that the use of contractions in formal writing is frowned upon. There’s no particular reason for this, other than contractions are a sign of laziness.
Abbreviations should also be avoided specifically for this reason. Is it really so difficult to write “Professor” rather than “Prof.”? When writing was a matter of cutting your own quills and stirring up the ink, or of saving inordinately expensive paper, there may have been an argument in favour of abbreviations; now, with computers and cheap recycled paper, I don’t think the argument would hold any water.
That said, there are a few situations where you’re allowed to be lazy:
* Credentials after names (e.g. M.D., B.A.), because they’re often short forms of Latin words which might take you several sentences to complete
* For the same reason, common abbreviations or acronyms which have been written out in full the first time they’re used in the paper (could you imagine writing deoxyribonucleic acid – DNA – a thousand times in a book?)
* Dates (e.g. 1066 A.D.)
If you are quoting something which uses abbreviations or acronyms, you must write it the way the original author wrote it; if the reference is vague, you might want to put in an editor’s note explaining the abbreviation in full.
Here are a couple of websites to get you thinking about such things:
If you’re following a particular format, they each have their own rules. Here are the OWL at Purdue’s pages for APA and MLA.
When in doubt, just write the words out in full; no one will criticise you for that.
