1. first 意为“先”“首先”，主要用来说明顺序，暗示接下去还有其他的动作或事件要发生，因此其后往往接有或暗示有 then, next, last 之类的词。如：
First think and then speak. 先想后说。
First, I must apologize to you. 首先，我要向你们表示歉意。

2. at first 意为“起初”“开始”，它主要强调前后对照，暗示接下去的动作与前面的动词不同甚至相反，因此其后通常有but, afterwards, soon, at last 等词语相呼应。如：
The work was hard at first, but I got used to it. 起初这活儿很累，不过后来我习惯了。
At first they were very happy, but then things started going wrong. 起初他们生活得很幸福，但后来就开始出问题了。
注意上，at first与at last不是一对反义词组，后者的意思是“终于”“最终”。如：
She has at last got everything ready. 她最后把一切准备好了。
At last the bus came. I had been waiting for half an hour. 最后公共汽车来了，我已等了半小时。