序数词前可加a。但需区别：
不定冠词（a/ an）+序数词 表示\"又一/再一\"的意思；
定冠词（the）+序数词 表示\"第几\"的意思。
This is the second time I have come here.
第二次来
The man had two houses in the country,and he bought a third one.
又买了一所房子。
