基数词＋单数名词＋形容词构成合成形容词.如：一个五岁的男孩a five-year-old boy；一座800米长的桥an 800-metre-long bridge；女子400米接力girls'400-metre relay race.16-year-old是形容词,比如说a 16-year-old boy,但是16 years old一般作表语,比如the boy is 16 years old.
2.基数词和名词构成合成词时误用复数.
My teacher is a twenty –five-years-old girl.
〔解析〕本句子是考查：基数词＋单数名词＋形容词构成的合成形容词的用法.中间的名词用单数形式,所以把twenty –five-years-old改为twenty-five-year-old.
希望对你有所帮助.
