The spelling of English compound nouns varies. They are sometimes one word and sometimes two. There are no rules to determine if they are one word or two.
Compound words written as one word often consists of two syllables. This is the case with words such as bookstore, textbook and football. However, words such as basketball, newspaper and sunflower have three syllables.
The word bookstore is one word, but shoe store is two. Though both compound nouns end with store, one is monosyllabic and one is not. The compound nouns hotcake and hot dog both begin with hot and have two syllables. However, one is one word and the other is two.
The compound noun nightclub is one word. However, health club is two. Though they both end with the word club, one is written as a single word and the other as two.
The compound noun grapefruit is one word, but passion fruit is two. The compound chocolate cake is two words, but cheesecake is only one. Both passion fruit and chocolate cake have three syllables. However, waterfall and rainfall are both single words even though waterfall has three syllables.
English has many compound nouns.The spelling of compound nouns varies. Unlike in languages such as German which write all compound nouns as one word, English does not. English compound nouns are sometimes written as one word and sometimes as two.