first有四个词形：adj.adv.n.pron.这里主要谈adj.和adv.
first adj.序数词 第一的
Example:He was the first student who arrived at the classroom.
adv.首先
First I fed the baby.Then I made myself a sandwich.
at first是一个介词短语在句中充当状语
首先
At first I was nervous,but I soon started to relax.
补充一个firstly 修饰句子的副词 首先
The plan was not practical,firstly because of the cost,and secondly because local people did not support it.
通常可互用,但有差别,需查词典体会.