表示顺序的词称为序数词。如first， second， third， fourth， ninth， twenty-first， forty-fifth等。

　　序数词可以用来表示日期和世纪。May the first / the first of May (5.1) ， August the eighth / the eighth of August (8.8) ， the twentieth century， the twenty-first century

　　序数词在句子中前面一般加the

　　The first of October is our National Day.

　　She was the third to arrive.

　　序数词在句子中前面加不定冠词，表示“再一”又一“。

　　They'll have to do it a second time.

　　Shall I ask him a third time?

　　When he sat down ， a fourth man rose to ask.