序数词的用法
①作主语 e.g. The third of the month was a holiday.
②作表语 e.g. Who was the third?
③作定语 e.g. We live on the fifth floor.
④作宾语 e.g. I was among the first to learn of this.
⑤作同位语 e.g. Who is that man, the first in the front row?
⑥作状语 e.g. When did you first meet him?
注：有时序数词并不指顺序，而是表示再一，又一的意思。此时序数词前要加不定冠词
e.g. I want to make a second try.