English grammar rule Missing hyphen

Hyphens are used to create compound words. In many cases, the compound word acts as an adjective within the sentence. However, hyphens aren’t necessary to join with very, or with an adverb ending in -ly (very nice clothes, strangely dressed woman). If you’re using an age or time period as an adjective before a noun, use two hyphens (The three-year-old dog), but you don’t need hyhens if the noun comes first (She is five years old). Both cardinal and ordinal numbers may be part of a compound word (one-time admission, first-year resident).

Examples of 'Missing hyphen' rule

Incorrect	James read a 400 page book last year.
Correct	James read a 400-page book last year.
Incorrect	Sarah worked at a coal fired plant.
Correct	Sarah worked at a coal-fired plant.
Incorrect	Her ten year old brother is smart.
Correct	Her ten-year-old brother is smart.