Use a hyphen...
with some prefixes and suffixes
1) To avoid multiple letters.
2) If the root word is capitalised.
3) With specific prefixes and suffixes.
self-sacrificing, all-seeing, ex-wife, vice-chairman, president-elect
4) To avoid ambiguity or awkward pronunciation.
un-ionised [unionised], re-read
5) Where a list of words each have the same prefix or suffix.
pre- and post-recession, over- and under-weight
to form compound words
1) For clarity.
sit-in, stand-out, Mother-In-Law
2) In compound adjectives that modify what they precede.
blue-chip company, devil-may-care attitude, up-to-the-minute news
with fractions, numbers and initial letters
1) With fractions and numbers between 21 and 99.
one-half, sixty-four, twenty-eight and three-quarters
2) Words that start with a capital letter
X-ray, T-shirt, U-Turn
to divide words at the right hand margin.