Use a hyphen to link words such as compound words and adjectives. Basically, if the words don’t make sense on their own, use a hyphen. For example: full-time faculty. Do “full faculty” and “time faculty” make sense? They don’t, so use a hyphen.
a nine-year-old girl
Omit the hyphen if the compound adjective follows the noun.
She is nine years old.
They work full time.
Some words are always hyphenated, like e-mail. However, some words are only hyphenated when they precede a noun and are used as an adjective.
Joe Davis lives on campus.
Joe Davis is looking for on-campus housing.
Susan Smith takes classes full time.
Susan Smith is a full-time student.
Adjunct faculty teach part time.
Some faculty members teach on a part-time basis.
There is no need for a hyphen after a word that ends in “ly.”
The college is a highly respected college.