When To Use A Hyphen

• A hyphen is used whenever a fraction is mentioned in words. E.g. two-thirds, one-fourth, one-sixth, etc.
• In the words which begin with prefixes such as ex, semi, pro, anti, non, neo, etc., a hyphen must be used right after the prefix. E.g. ex-wife anti-aircraft, anti-fascist, anti-submarine, non-combatant, non-existent. However, the exceptions to this rule are many and include commonplace words like antibiotic, anticlimax, antidote, antiseptic and antitrust among others.
• In Nouns which are formed with prepositional verbs hyphen must be used. E.g.  Bail-out, set-up, shake-up, build-up, call-up, get-together, lay-off, pay-off, etc. However, if these words are used as verbs then hyphens must not be used.
• If an adverb which doesn’t end in -ly is used as a compound word, in front of a noun then a hyphen must be used. E.g., ‘The well-known man appeared before the crowd.’ Whereas, if the compound word is placed after the noun, a hyphen must be avoided. E.g., ‘The man who appeared before the crowd was well known.’ In this sentence, since the words ‘well known’ follow the noun they describe, no hyphen is used.
• If a complex word is formed with two distinctive words, and if the last and the first letters of the two words are the same, a hyphen must be used. For e.g., book-keeping, coat-tails. Also, when the last letter of the prefix and the first letter of the word are same, hyphens must be used. For e.g., co-operate, pre-eminent, pre-empt, re-emerge, re-entry, trans-ship, etc. The exceptions to this rule are override, overrule, underrate, withhold etc; these have been accepted without hyphens owing to mass usage.
• Hyphen is used in some typical words such as the words starting with “all” e.g. all-powerful, all-knowing, all-encompassing etc.
• Hyphen is also used in some typical words such as words staring with “half” e.g. half-asleep, half-court, half-moon, half-wit etc.
• Hyphens are also used in some typical words such as with words staring with “self”. For e.g., self-inflicted, self-control, self-righteous, etc.
• Even words starting with “semi” require a hyphen. For e.g., semi-final, semi-skilled, semi-automatic etc.
• Similarly, words starting with “quasi” are also considered incorrect when not used with a hyphen. For e.g., quasi-judicial, quasi-socialists, quasi-religious etc.
• Avoid using a hyphen in words and pronouns compounded with self. For e.g., selfless, selfish, myself, itself, herself, etc.
• It is extremely essential to use a hyphen in compound adjectives while describing ages and even lengths of time, to avoid ambiguity. For example, 50-year-old trees means trees which are 50 years old, whereas 50 year old trees might imply 50 trees each a year old.
• In adjectives formed by two or more words, hyphens must be used. For e.g., right-wing groups, balance-of-payments difficulties, private-sector wages, public-sector borrowing requirement, a 70-year-old judge, state-of-the-union message, value-added tax (VAT). Adding a hyphen here makes us understand the correct meaning of the words. For e.g., ‘state of the union message’ looks like condition of the message given by a union whereas ‘state-of-the-union message’ stands for a message issued by the state headed by the union.
• Hyphen must be used in compound numerals. For e.g., twenty-two, forty-second, thirty-five thousand, etc.
• A hyphen is also used in compounding numerals with other words. For e.g., ten-foot post, twelve-o’clock lunch break, 50-yard dash, four-year-old boy.
• A hyphen should also be used in certain compounds made up of nouns and prepositional phrases. For e.g., mother-in-law, hand-in-hand, off-the-cuff, etc. 
• Hyphens are also used to indicate a break of some kind that appears in a text, such as a line break on a page. For e.g., "Albert was concerned his earlier actions would conse-
  quently land him in hot water." The hyphen here means that the word continues on the next line. In this case, hyphens should split the word evenly at syllable boundaries.
• However, there are some compound words which are so common and so frequently used that they have become "permanent compounds." For combinations like racehorse, Boy Scout basketball, railroad, prime minister, vacation home, know-it-all, sit-in, city-state, high school, break-in and breakout etc, it is better to refer a current dictionary to learn whether to separate them, hyphenate them or write them as one word. Dictionaries may differ on permanent compounds; however, at least the presence of one form or another in an authoritative dictionary should be enough to provide a justification for the style the writer finally settles on.