前者,··有连字符day不需要加s·
同理·I am 21 years old.= I am 21 year-old.
There are 28 days in February.
February usually have twenty-eight days.
Febuary usually has twenty -eight day 注意:连字号后面不能加s
twenty-eight days对
There are 28 days in February或There are twenty-eight days in February