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.