Some helpful hyphen rules follow:

1. Hyphenate a series of words used as a single unit, especially two or more words used as a single adjective. Examples:

  • the eighteenth century = a noun referring to the period 1700-1799
  • eighteenth-century = an adjective applied to something from or about the period
  • eight years = a period of time
  • eight-year-old children = children who are eight years old
  • eight year-old children = eight children who are one year old

2. Note that many style guides (including the MLA) say that adverbial phrases should not be hyphenated: “Well known play” takes no hyphen in MLA style because “well” is an adverb.