1．在某些习惯用语以及专用名词前。例如：
at first起先，first of all首先、第一，first or last迟早，
from first to last自始至终，First Lady（美国）总统夫人
2．序数词用作副词时。例如：
His name stands first on the list. 他名列榜首。
The Temple of Heaven was first built during the Ming dynasty. 天坛始建于明朝。
3．序数记号为名词所有格、物主代词或形容词等所限定时。例如：Fortunately for him，his father's second wife was kind to him too.他很幸运，他的继母对他也很慈爱。
My first impression of London is of rain and fog and people with umbrellas. 伦敦给我的第一印象就是雨、雾和带着伞的人们。
4．由序数词与名词构成的复合形容词。例如：
They are all second-raters. 他们都是些平庸之辈。
5．序数词用作名词复数时。例如：These are all firsts. 这些都是一等品。