Conventions of Composition Rule 124

Rule: When using two or more words as a single adjective, hyphenate. Hyphenate a noun and a participle, an adjective and a participle, two nouns, or two adjectives, as long as together they form one descriptor. If two nouns, used together, form a verb, hyphenate. Note: if one of the words is an adverb, do not hyphenate.

Examples:

Correct: I heard a heart-rending scream come from the strange-looking creature.

Correct: Not to touch white-hot coals seems self-evident to me.

Practice putting hyphens where they should go in the following sentences:

1. We wanted to purchase the seven year old car because it was dirt cheap.

2. The top of the line model costs more than the new to me junk heap he tried to sell me.

3. We want to air condition the larger than life English Wing in the Main Building.

Resources for further explanation of using hyphens:

Grammar Book's Hyphens

Oxford Living Dictionaries Hyphens