When two or more words are combined together make one modifier (or adjective) placed before a noun, a hyphen must be used between the words to connect them.
Example:
Her one-month-old baby waves at her.
My not-so-handsome brother married a fashion model.
Since in English adjectives don't become plural, the compound modifier will always remain singular.
Example:
1) We saw twelve-foot waves in the ocean.
2) The waves we saw were at least twelve feet high.
Note that in the first sentence (1) you don't say twelve-feet waves even though twelve is more than one and requires a plural noun after it. In the second sentence (2), however, twelve feet is correct because it comes after the noun.
This is particularly important because many compound modifiers begin with a number. Whether the number is spelled out or in numerals and whether it is cardinal or ordinal, this rule always applies: the noun in the compound remains singular and the compound is hyphenated.
Example:
Her six-month-old baby waves at her.
I've read many nineteenth-century works.
I've read many 19th-century works.
She's doing her 3-day diet.
