First 可當形容詞和副詞用，前者意為「第一的；最早的；最初的；最先的」，而後者意為「第一；首先；首次；最先；最初」。例如：
形容詞
- Your first year at university can be quite frightening and exciting at the same time. (你上大學的頭一年可能恐懼和刺激兼而有之)
- Why didn’t you seek help from the police in the first place? (你為何不先求助於警方呢？)
副詞
- You (go) first! (你先請！)
- When did the idiom first appear? (這個成語首次出現在什麼時候？)
- He finished first in math, second in English, and third in fine arts. (他獲得數學第一名、英文第二名、美術第三名)
Firstly 僅當副詞用，意為「第一；首先」，與當副詞用的 first 同義，但 firstly 比 first 來得正式。寫作時，我們經常使用 first 或 firstly 來做列舉，即第一、第二、第三… 等等，英文為 first, … second, … third … 或 firstly, … secondly, … thirdly … (美式英語還使用 first of all, … second of all, … thirdly …)。例如：
- First let us discuss the most important issue. (首先讓我們來討論最重要的問題) (正)
Firstly let us discuss the most important issue. (正)
At first let us discuss the most important issue. (誤) - There are three diseases to take into account – firstly hyperthyroidism, secondly glaucoma, and thirdly osteoporosis. (有三種疾病要考慮 － 第一是甲狀腺機能亢進，第二是青光眼，第三是骨質疏鬆症) (正)
There are three diseases to take into account – at first hyperthyroidism, secondly glaucoma, and thirdly osteoporosis. (誤) - Every employee wants two things from their boss: Firstly, a good salary, and secondly, a large year-end bonus. (每個員工都想從他們老闆那裡得到兩樣東西：第一，可觀的薪水，第二，一大筆年終獎金) (正)
Every employee wants two things from their boss: First a good salary, and second a large year-end bonus. (正)
Every employee wants two things from their boss: At first a good salary, and second a large year-end bonus. (誤)
從上面的例句可知，at first 與 first 或 firstly 不能劃上等號。這個片語意為「起初，起先，當初」(= at/in the beginning)，係用於對比。例如：
- At first, I thought she was shy, but then I discovered she was a lively, talkative person. (起初我還以為她很靦腆，後來才發現她是個活潑健談的人)
- At first John thought I was joking but then he realized I meant it. (起初約翰以為我在開玩笑，後來他才意識到我是認真的)
- The old man fell down and cried for help. No one was interested in him at first, but eventually two girls came to help him. (那個老人跌倒後呼救。起初沒有人理他，但最後有兩個女生前來幫他)