English often combines two or more words, or a word and a combining form, to make a compound word — but it doesn’t do it in a consistent way.

Some compounds are closed:

bookstore, coworker, daylight, fatherland, fireball, nationwide, nonsmoking, steelworker

Some compounds are open:

book club, coffee shop, day labor, fire fighter, mass media, mother ship, steel wool

And some compounds use hyphens:

co-owner, father-in-law, fire-eater, mass-produce, mother-of-pearl, nation-state, self-governing

To make things worse, words that are not compounds may need to be hyphenated when you use them together as an adjective that comes before a noun:

an English-class assignment