Compound words in English are formed by combining two or more words, usually nouns or adjectives. There are three forms of compound words: closed form with the words joined together like "firefly"; hyphenated form like "daughter-in-law"; and open form with separate words like "post office". Compound nouns are formed by joining a noun modified by another noun or adjective, such as "toothpaste" from "tooth + paste". Compound adjectives are made up of two or more words usually joined by hyphens, like "English-speaking".