1．在某些習慣用語以及專用名詞前。例如：

at first起先，first of all首先、第一，first or last遲早，

from first to last自始至終，F(xiàn)irst Lady（美國）總統(tǒng)夫人

2．序數(shù)詞用作副詞時。例如：

His name stands first>The Temple of Heaven was first built during the Ming dynasty. 天壇始建于明朝。

3．序數(shù)記號為名詞所有格、物主代詞或形容詞等所限定時。例如：Fortunately for him，his father's second wife was kind to him too.他很幸運，他的繼母對他也很慈愛。

My first impression of London is of rain and fog and people with umbrellas. 倫敦給我的第一印象就是雨、霧和帶著傘的人們。

4．由序數(shù)詞與名詞構(gòu)成的復合形容詞。例如：

They are all second-raters. 他們都是些平庸之輩。

5．序數(shù)詞用作名詞復數(shù)時。例如：These are all firsts. 這些都是一等品。

（來源：網(wǎng)絡）