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.
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