1.序数词主要用作定语，前面一般要加定冠词(或物主代词)：
Tom is their second son.
They celebrated the 10th anniversary of the founding of the republic.
I will never give up, not even on the 1,000th or 10,000th try.
2.有时前面可以加一个不定冠词来表示“再一”， “又一”这样的意思：
We’ll have to do it a second time.
Shall I ask him a third time?
When I sat down, a fourth man rose to speak.
3.First,second 等有时可用来表示“第一个人(批)”“第二个人”等：
She was among the first to come and settle in Dujiashan.
You will be the second to speak.
4.如果数字较长，序数词总避免使用，而且读的方法也常常简化：
第201房间： Room 201
第319面： page 319
第一拖拉机厂：the Number I Tractor Works
第六号车厢：Carriage No.6
南京路1490号：1490 Nanjing Road
电话号码55――2347：telephone number : 55—2347
5. 在谈编了号的东西时，我们可以用基数词表示顺序：
the first part------part one
the third squad------squad three
the twenty-third section----section twenty-three