How to Write a Compound Adjective

18 year old is an adjective made from three words joined together with hyvens. The rule is always use hyphens in numbers from 21 to 19, ine 20 hyphen One, 290, hypen Nine. In a phrase like the politicians were well respected and their attitudes are out of date, we don't need a hyphen. Most compound nouns are written as two separate words - if you're not sure, check in a good dictionaryts.