Writing compound nounsMany compound nouns are written as one word, but some are written with hyphens or spaces. In modern English, hyphens are less common than they were in the past. A good learner’s dictionary will tell you how each compound is usually written.
Examples of one word
bathroom sheepdog windscreen
Examples of hyphens
check-in fire-fighter son-in-law
Examples of spaces
bottle opener New Year’s Day Prime Minister