a an 是不定冠词 ,用在泛指某个东西的时候.a用在辅音前,an用在元音前如 This is a desk.这是一张桌子He ate down an apple just now.他刚才吃下一只苹果.the 是定冠词,用在特指某个或某些东西的时候.There was an ap...