Hyphen or not
When two separate words are joined to become one, you need to use a hyphen. Those joined words then become an adjective. Look at these examples:
(a) as a single adjective before a noun
- This is a two-way traffic, so drive carefully.
- Dan Brown became a well-known author when he wrote “Da Vinci Code”.
- I attended a boring three-day training programme.
(b) to indicate numbers
- We have thirty-two students today.
- The man who passed away was only forty-two.
(c) after a prefix
- Donald Trump is a self-made millionaire.
- In the pre-dawn attack, many were killed.
- My neighbour is an ex-policeman.
- There will be a big sale in mid-year.
Words that are joined to become an adjective do not have plural forms, so it is wrong to say:
- This is a two-ways traffic, so drive carefully. (X)
- I attended a boring three-days training programme. (X)