一、从词性上看区别
first可用作形容词或副词，有时还可用作代词;而at first作为介词短语，只起副词用作用(在句中用作状语)。如：
First class is the most expensive way to travel. 坐头等舱是最贵的旅行方式。(first为形容词，在句中用作定语)
This is the first time I have heard of such things. 这是我第一次听到这样的事。(first为形容词，在句中用作定语)
You needn’t read the whole book hut you must read the first four chapters.你们不必把整本书全读完，但必须读完前四章。(first为形容词，在句中用作定语)
I asked them to ring first in case we were out. 我请他们先打电话来，以防我们不在家。(first为副词，在句中用作状语)
She’s always the first to arrive and the last to leave. 她总是第一个来，最后一个离开。(first为代词，可视为其后省略了one, person等之类的词)
His second suggestion was not much better than his first. 他的第二个建议比第一个好不了多少。(first为代词，可视为其后省略了suggestion)
At first we used hand tools. Later we had machines. 开始我们用手工工具，后来我们有了机器。(at first在此用作状语)
二、从用法上看区别
1. first的用法
first 用来说明顺序，意为“先……”，暗示接下去还有其他动作或事件要发生，因此其后往往接有(或暗示有) then, next, last 等词。如：
Think first, then act. 先想清楚再行动。
I’ll have to finish my homework first. 我得先把作业做完。
First (you) boil some water. Then (you) warm the teapot. Then (you) add three teaspoons of tea. Next, (you) pour on boiling water... 你先烧些开水，然后把茶壶烫热，接着放三勺茶叶，随后冲入开水……
John came home from work. First he read the paper for a while, then he got up from the chair and turned on the radio. 约翰下班回家，先看一会儿报，然后从椅子上站起来，打开收音机。
2. at first 的用法
at first 的意思是“起初”“开始”，它主要用于强调前后对照，暗示接下去的动作与前面的动作不同甚至相反，因此常有 but, afterwards, soon, at last 等相呼应。如：
At first I didn’t want to go, but I soon changed my mind. 我开始不想去，但我很快就改变了主意。
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。
At first I paid little attention, but slowly my interest awoke. 起初我没怎么注意，但逐渐产生了兴趣。
三、其他几点区别
1. at first 除以上用法外，其他场合一般不用它;而first除表示“先……(然后……)”外，还可表示“最先”“第一次”“首先”等。如：
Ladies first. 女士优先。
That’s mine—I saw it first. 那是我的，是我先看见的。
When did you first meet him? 你第一次见他是什么时候?
2. 有时at first并非固定搭配(first后还修饰有其他词语)。如：
She was so nice a girl that he fell in love with her at first sight. 她是这样好的一个姑娘，使他一见钟情。
She could tell at first glance to which class a man belonged. 她一眼就能看出一个人属于哪个阶级。
3. at first与at last不是一对反义词组，后者的意思是“终于”“最终”。如：
At last the truth became known. 最后真相大白了。
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。
