一、從詞性上看區別

first可用作形容詞或副詞，有時還可用作代詞;而at first作為介詞短語，只起副詞用作用(在句中用作狀語)。如：

First class is the most expensive way to travel. 坐頭等艙是最貴的旅行方式。(first為形容詞，在句中用作定語)

This is the first time I have heard of such things. 這是我第一次聽到這樣的事。(first為形容詞，在句中用作定語)

You needn’t read the whole book hut you must read the first four chapters.你們不必把整本書全讀完，但必須讀完前四章。(first為形容詞，在句中用作定語)

I asked them to ring first in case we were out. 我請他們先打電話來，以防我們不在家。(first為副詞，在句中用作狀語)

She’s always the first to arrive and the last to leave. 她總是第一個來，最后一個離開。(first為代詞，可視為其后省略了one, person等之類的詞)

His second suggestion was not much better than his first. 他的第二個建議比第一個好不了多少。(first為代詞，可視為其后省略了suggestion)

At first we used hand tools. Later we had machines. 開始我們用手工工具，后來我們有了機器。(at first在此用作狀語)

二、從用法上看區別

1. first的用法

first 用來說明順序，意為“先……”，暗示接下去還有其他動作或事件要發生，因此其后往往接有(或暗示有) then, next, last 等詞。如：

Think first, then act. 先想清楚再行動。

I’ll have to finish my homework first. 我得先把作業做完。

First (you) boil some water. Then (you) warm the teapot. Then (you) add three teaspoons of tea. Next, (you) pour on boiling water... 你先燒些開水，然后把茶壺燙熱，接著放三勺茶葉，隨后沖入開水……

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. 約翰下班回家，先看一會兒報，然后從椅子上站起來，打開收音機。

2. at first 的用法

at first 的意思是“起初”“開始”，它主要用于強調前后對照，暗示接下去的動作與前面的動作不同甚至相反，因此常有 but, afterwards, soon, at last 等相呼應。如：

At first I didn’t want to go, but I soon changed my mind. 我開始不想去，但我很快就改變了主意。

The work was hard at first, but I got used to it. 起初這活兒很累，不過后來我習慣了。

At first they were very happy, but then things started going wrong. 起初他們生活得很幸福，但后來就開始出問題了。

At first I paid little attention, but slowly my interest awoke. 起初我沒怎么注意，但逐漸產生了興趣。

三、其他幾點區別

1. at first 除以上用法外，其他場合一般不用它;而first除表示“先……(然后……)”外，還可表示“最先”“第一次”“首先”等。如：

Ladies first. 女士優先。

That’s mine—I saw it first. 那是我的，是我先看見的。

When did you first meet him? 你第一次見他是什么時候?

2. 有時at first并非固定搭配(first后還修飾有其他詞語)。如：

She was so nice a girl that he fell in love with her at first sight. 她是這樣好的一個姑娘，使他一見鐘情。

She could tell at first glance to which class a man belonged. 她一眼就能看出一個人屬于哪個階級。

3. at first與at last不是一對反義詞組，后者的意思是“終于”“最終”。如：

At last the truth became known. 最后真相大白了。

She has at last got everything ready. 她最后把一切準備好了。

At last the bus came. I had been waiting for half an hour. 最后公共汽車來了，我已等了半小時。