Only 可用作形容詞和副詞。
Only 當形容詞時意為「唯一的，僅有的」，其後須接名詞 (當然了，only 和名詞之間可以插入數字或其他形容詞) 或 one。例如：
- Mary is the only girl in the class. (瑪麗是班上唯一的女生)
- There are only three books on the desk. (書桌上只有三本書)
- That was the only off-the-rack T-shirt left in that color. (那是那種顏色僅剩的一件均碼T恤) － off-the-rack (adj.) 為美式英語，意為「(衣服) 均碼的，標準碼的」；英式英語為 off-the-peg，澳洲英語叫做 off-the-hook。
- John is the only one who can speak Indonesian in the class. (約翰是班上唯一會講印尼語的人) (正)
John is the only who can speak Indonesian in the class. (誤)
值得一提的是，在「one of + 複數名詞 + 關係代名詞 + S + V」的句型中，關代的先行詞是複數名詞、不是 one，所以關係子句中的動詞要用複數動詞。然而，若 one 的前面加上 the only，那麼關代的先行詞就變成了 one，亦即關代後面要用單數動詞。例如：
- Ted is one of the students who are absent. (泰德是缺席的學生之一) (正)
Ted is one of the students who is absent. (誤) - Ted is the only one of the students that is absent. (泰德是唯一缺席的學生) (正)
Ted is the only one of the students that are absent. (誤)
Only 當副詞時意為「只，僅，才」。例如：
- It’s only four o’clock and dawn is already breaking. (才 4 點，天就開始亮了)
- Amy only arrived fifteen minutes ago. (艾美 15 分鐘前才到)
- I was only joking. (我只不過是在開玩笑)
- Only an idiot would do that. (只有白癡才會做那種事)
- Only Tom and I attended almost every lecture and seminar when we were college students. (我們唸大學的時候，只有湯姆跟我幾乎參加了每一場講座和研討會)
- At present these bicycles are only available in Taiwan. (目前這種腳踏車只有在台灣才能買到)
在口說英語中，only 可與 just 連用來加強語氣。Only just 意為「剛剛，才；差一點沒，幾乎不」。例如：
- I’ve only just moved to Taipei. (我剛剛搬到台北)
- People were leaving and he’d only just arrived. (人們都要離開了，而他才剛剛到)
- There were only just enough reference books to go round. (參考書剛好夠每人發一本)
- She only just caught the bus. (她差點沒趕上公車)
- I got there in time for my flight, but only just (= but I almost did not). (我及時趕上了班機，差一點就錯過了)
Only 當副詞時可放在句中不同的位置，視其所要強調的元素而定。若主詞是重點所在，那麼就把 only 放在主詞的前面。例如 (援用上面的例句)：
- Only an idiot would do that. (只有白癡才會做那種事)
- Only Tom and I attended almost every lecture and seminar when we were college students. (我們唸大學的時候，只有湯姆跟我幾乎參加了每一場講座和研討會)
如果重點是句中其他元素，那麼 only 跟一般副詞一樣， 通常位在句子的中間，亦即位在主詞和主動詞之間 (即普通動詞之前)，或位在語氣助動詞或第一個助動詞之後，或位在當主動詞的 BE 動詞之後。例如 (援用上面部分例句)：
- Amy only arrived fifteen minutes ago. (only 位在主詞和主動詞之間)
- Amy had only arrived fifteen minutes before I left for the airport. (艾美在我動身前往機場之前 15 分鐘才到) (only 位在第一個助動詞之後)
- It’s only four o’clock and dawn is already breaking. (only 位在當主動詞的 BE 動詞之後)
如果要強調的重點是整個子句，那麼 only 就放在子句之前。例如：
- They want to have their first child only when they are ready. (只有在他們準備好的時候才會想要生第一個孩子)
- Only when his husband ends his extramarital affair will there be probably any improvement in their relationship. (只有他先生結束婚外情，他們的關係才可能會有所改善) － Only when 引導的副詞子句挪到句首時，主句要倒裝。