Some countries have single-sex education models, while in others both single sex and mixed schools co-exist and it is up to the parents or the children to decide which model is preferable.
This is a sentence from a book. I would like to ask that why first 'single-sex' is used and then 'single sex'? Thank you!
In both scenarios, the phrase is used as a compound adjective. The rules for hyphenating compound adjectives are somewhat loose - usually, only hyphenate when clarification is needed.
If the author had omitted the hyphen in the first example, you might read it as "single sex education models," which would be logically possible but not what the author means. In the second example, "single sex ... schools," there is no third adjective, and the phrase has (hopefully) only one logical meaning.
Adjectives of more than one word (compound adjectives) require hyphens when used before the noun. Sometimes a compound word is used enough that over time it merges into one word, but two separate words is incorrect for a single adjective. Note, though, that some things such as very ... are not compound adjectives. That would be an adverb with an adjective. Also, two different adjectives applied to the same noun should not have hyphens. In conclusion: the second sentence is grammatically incorrect.
