Some compound words are linked by a hyphen. There is no simple rule to help know which compounds need hyphens and which do not. Hyphens are, however, being used less and less, especially in compound nouns. People tend to write website rather than website, and air raid instead of air-raid. Hyphens are still often used:
 To form a verb from a compound noun:
A booby trap
The area was heavily mined and booby-trapped.
 To form a noun from a phrasal verb or prepositional verb:
I don’t want you to build up your hopes and then be let down.
A build-up of harmful gases.
 To form a word with a prefix:
Co-opt multi-storey post-war.
 To form some compound adjectives:
Easy-care head-on right-handed.
Don’t use a hyphen between the two parts of a phrasal verb:
Time to top up your mobile phone.
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