Usually words with prefixes are written as one word. (unhappy, disabled, reabsorb – the prefixes are underlined.) New prefix-words and less common words will often hyphenate a prefix to avoid confusion. Originally the idea of e-mail was new and was hyphenated to make it clear that it was electronic mail. Today it has become very common and there is no longer any need to hyphenate email.
The most commonly hyphenated prefixes are: anti-, e-, ex-, non-, pre-, post-, pro-, re- and semi-
He is anti-internet and does not own a computer.
Murray MacLehose is an ex-governor of Hong Kong.
We will have an e-audit tomorrow.
I want a non-smoking seat.
Pre-revolutionary China was torn apart by many wars.
There has been more democracy in post-1997 Hong Kong.
He is pro-China.
I will have to re-explore the caves, since the maps were lost.
She is semi-retired.
Many of these words are simply cases where the whole word would look too long and clumsy or have too many vowels running together if unhyphenated. If the prefix ends and the word begins with similar sounds then the combination is often hyphenated.
Many anti-intellectual people don’t understand university.
Usually it is better to have fractions hyphenated in text, if the fraction is a prefix. Non-prefix fractions are not hyphenated.
half of a kilometre
one and six-tenths
I ate two-thirds of the cake.
I ate two pieces of the cake that was cut into thirds.
As well as using prefixes, when two or more words are used to describe another word, hyphens are sometimes used. (Not always like some teachers claim.) Normally the words would not be hyphenated. Using a hyphen makes it clear that the two words together describe the third word.
Low-level radioactive waste.
The water in the lake is at a low level.
The up-to-date records were given to the principal.
The records are up to date.
Describing words that end in – ly (adverbs) are not normally hyphenated.
He is a happily married man.
To hyphenate a prefix or not is the writer's choice. The things to consider are which form is most common and which choice makes your writing clearer. The only real rule is that it is better style to remain consistent, use only e-mail or email not both.
Next week I'll cover some other rules and suggestions about using compound words.