不定冠词a/an可以表示数量为一,但和one有区别,a和an强调事物类属,而one强调数量。例如: The child drew a big circle on the paper. The road is wide enough for a car to pass through. (强调路的宽度够小汽车通过,而不是够别种车辆通过) The road is wide enough for one car to pass through. (强调路的宽度够一辆汽车通过,而不够两辆通过) I have a diskette which is a megabyte large. (强调软盘的容量是一兆字节,而不是一千字节或一吉字节) I have a diskette which is one megabyte large. (强调软盘的容量是一兆字节,而不是两兆字节) 希望我能帮助你解疑释惑。