first可用作形容词或副词,有时还可用作代词;而at first作为介词短语,只起副词用作用(在句中用作状语)。如: First class is the most expensive way to travel. 坐头等舱是最贵的旅行方式。(first为形容词,在句中用作定语) This is the first time I have heard of suc...