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。