two-year-old boy,two-year-old做形容词,但是不能做表语,即不能说成the boy is two-year-old .
不用连词符号的就不能做形容词了,只能说成,the boy is two years old.
你只要记住,不用连词符号时,year的单复数根据前面的数字来变化..
用连词符号时,year都是用单数,不管前面的数字.