序数词在句中可作主语、宾语、定语和表语。

　　The second is what I really need.(作主语)

　　He choose the second.(作宾语)

　　We are to carry out the first plan.(作定语)

　　She is the second in our class.(作表语)

　　★序数词前常要加定冠词 the;但若序数词前出现不定冠词a或an时,则表示“再一”“又一”。

　　We’ve tried it three times.Must we try it a fourth time 我们已经试过三遍了还必须试一次(第四次)吗

　　▲表编号 结构:名词(首字母要大写) + 基数词 = the +序数词 +名词

　　Lesson One = the first lesson

　　the fifth page=Page 5(five)

　　the twenty-first room=Room 21(twenty-one)