EXPLANATION There are countless words in English that have been formed by attaching a prefix to the beginning of the word: e.g. prehistoric, international, extraordinary, hyperlink. Such words often start their lives with a hyphen after the prefix but once the word is in general use, the hyphen often gets dropped. Perhaps the fastest transition in history happened with the word e-mail, which very rapidly became email.
When, then, should we use hyphens? Firstly, if the use of a prefix means that the same vowel is repeated, a hyphen eliminates awkwardness: pre-election, re-engineered, pre-existing, re-enter, de-emphasize. (You will come across a few words with double vowels where it is widely accepted that the hyphen is not essential: cooperate, coordination - though it is not wrong to write co-operate, co-ordination).
Of course, spelling can appear awkward even if different vowels are adjacent, so a hyphen should be used then as well: de-ice, anti-aircraft, semi-official. In fact, those words just show what hyphens do anyway - they link concepts: pro-government, re-allocate, anti-uranium, pre-World War II, post-Elizabethan, post-1945, post-9/11. (While reallocate might be accepted by a spell-checker, progovernment looks decidedly odd and a hyphen is definitely needed before capitalised concepts such as World War II and dates.)
Finally, sometimes if the hyphen is not used, the word means something totally different! Study these pairs of sentences and work out why hyphens are essential to the hyphenated words.
- If you take your medicine, you will recover.
You ought to re-cover this book. - After five days, the searchers were recalled.
When some boys arrived late, the teacher decided to re-call the roll. - He was a reformed character: he no longer drank or gambled.
The skydivers broke their pattern and then re-formed the circle.