複合名詞

在英語中，字 (尤其是形容詞和名詞)以多種方式結合成複合結構。複合字一旦形成，它們有時會隨時間而變形。一個常見的模式是，兩個字 -- 譬如說，fire fly -- 經過一段時間後 以連字號結合在一起 -- fire-fly -- 然後又結合成一個字 -- firefly。我們只有一個方法可以知道英語複合字的拼字，那就是查閱權威的字典。
複合字有三種型態：

封閉型態 (closed form)，如 firefly, secondhand, softball, childlike, crosstown, redhead, keyboard, makeup, notebook。
加連字號型態 (hyphenated form)，如 daughter-in-law, master-at-arms, over-the-counter, six-pack, six-year-old, mass-produced。
開放型態 (open form)，如 post office, real estate, middle class, full moon, half sister, attorney general。

修飾用的複合字經常被加上連字號以避免混淆。紐約公立圖書館的「寫作者指南」(Writer's Guide) 指出，an old-furniture salesman 顯然是從事古家具的交易，但 an old furniture salesman 必定是個老人。然而，我們對 a used car dealer 可能就不會有這樣的混淆了。當複合修飾語位在名詞的前面時，它們通常加連字號：part-time teacher, fifty-yard-wide field, fire-resistant curtains, high-speed chase。然而，當這些相同的修飾用字位在名詞的後面時，它們不加連字號：: a field fifty yards wide, curtains that are fire resistant, 等等。The second-rate opera company gave a performance that was first rate。
形容詞的比較級和最高級在與其他修飾語複合時加連字號：the highest-priced car, the shorter-term loan。但這並非亙古不變的定律：the most talented youngster。以 ly 做字尾的副詞在與其他修飾語複合時不加連字號：a highly rated bank, a partially refunded ticket, publicly held securities。
有時加連字號的修飾語在變成複合名詞時，其連字號自動消失：A clear decision-making process was evident in their decision making. The bluish grey was slowly disappearing from the bluish-grey sky。但這也並非總是如此：your high-rise apartment building is also known as a high-rise。
當以一個人的年齡來修飾他或她時，該複合片語加連字號：my six-year-old son。但當年齡位在人的後面時，我們不加連字號：My son is six years old. 然而，He is a six-year-old.