The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 2, 'Spelling and Usage'
Hyphens should be used only when they have a specific purpose. They occasionally occur within the body of a word, particularly with re- followed by e (e.g. re-echo, re-enter), but they normally indicate that two or more words are to be read as a single word with only one main stress. The examples given below show forms that are attributive and have a single main stress and are therefore hyphenated, while predicative and other forms having two main stresses are not hyphenated:
a well-known fact the facts are well known
a tenth-century manuscript in the tenth century
Nevertheless, to avoid a proliferation of hyphens and where there is no possibility of ambiguity, forms such as a late eighteenth-century novelist, post-Second World War difficulties, are to be preferred to a late-eighteenth-century novelist, post-Second-World-War difficulties.
In phrases such as pre- and post-war governments, pro- and anti-abortion movements, eighteenth- and nineteenth-century literature, where two or more parallel hyphenated terms are combined, the first hyphen is followed by a space.
Adverbs ending in -ly and other polysyllabic adverbs are not hyphenated to a following adjective or participle:
a highly contentious argument
a recently published novel
a handsomely bound volume
a frequently occurring mistake
a hitherto unrecognized custom
ever increasing quantities
Collocations of certain monosyllabic adverbs (in particular ill but not well — see above) and a participle often have only one main stress and are therefore hyphenated even when used predicatively:
He is very ill-tempered.
Such a course of action would be ill-advised.
These prejudices are deep-seated.
Note that, unlike the words early, late, north, south, etc., the prefix mid- always requires a hyphen (except where it forms part of a single word, as in midnight):
The boat sank in mid-Atlantic.
a mid-June midnight flight
a mid-sixteenth-century chair
until the mid-nineteenth century
The presence or absence of a hyphen is often significant:
two-year-old dogs two year-old dogs
a deep-blue lake a deep blue lake
a vice-chancellor the vice squad
to re-cover to recover
There is considerable variation in the use of hyphens. Usage shifts over time and forms that were once entirely acceptable may now seem odd or old-fashioned. Some words that used to be hyphenated have now become so common that they are regarded as single unhyphenated words:
battlefield, bookshelf, paperback, subcommittee, subtitle
In short, if a compound is in frequent use and is pronounced as a single word it is usually acceptable to write it as one word without a hyphen. The best advice is to use a good dictionary and to be consistent.