Once 可當副詞、連接詞和名詞用。
Once 當副詞時意為「一次，一回」，如 We’ve met only once. (我們只見過一次面)。在這項用法中，我們經常使用 once a + 單數時間詞 (once a week/month/year etc. － 一星期/一個月/一年等一次) 及 once every + 複數時間詞 (once every two days/three years etc. － 每兩天/三年等一次) 來表示事情發生的頻率。例如：
- We go for dinner together once a week. (我們一個禮拜會共進晚餐一次) (正)
- We go for dinner together once the week. (誤)
- Tom visits his parents only once every two or three months. (湯姆每兩三個月才探望他父母親一次)
Once 當副詞時還意為「從前，曾經」。在這項用法中，once 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：
- John once knew Mary, but they are no longer friends. (約翰以前認識瑪麗，但他們已經不是朋友了)
- It was hard to believe that she had once been beautiful. (很難相信她以前很漂亮)
- Egypt was once a great nation. (埃及曾經是個偉大的國家)
由當副詞用的 once 所構成的片語不在少數，其中之一就是大家所熟悉的 once upon a time。這片語意為「從前，很久以前 (童話故事的開頭)」，如 Once upon a time there was a little girl called Little Red Riding Hood … (很久很久以前，有個小女孩叫做小紅帽…)，但它現在也不時用於口語中，意為「以前，過去」(long ago)，如 We used to go to nightclubs once upon a time. (我們以前常上夜店)。
Once 當連接詞時意為「一旦，一 … 就 …」(as soon as)。例如：
- Once I’ve picked Cindy up at the airport, I’ll call you. (一旦我在機場接到辛蒂，我就會打電話給你)
- Once you get into a bad habit, you’ll find it hard to get out of it. (一旦染上惡習，就很難改掉了)
與 when, while, before, after, by the time, as soon as, if 和 unless 等所引導的時間副詞子句一樣，once 所引導的時間子句亦不可使用表示未來的 will 或 be going to。例如：
- Once I pass all these exams, I’ll be fully qualified. (一旦我通過所有這些考試，我就完全合格了) (正)
- Once I will pass all these exams, I’ll be fully qualified. (誤)
Once 當名詞時意為「一次，一回」，主要用於 at once (立即，馬上；同時)、all at once (突然，忽然)、(just) for once (僅此一次，就這一回，難得一次)、(just) this once (就這一次，就此一次)、(just) that once (就那一次) 及 (just) the once (就這一次；就那一次) 等片語中，或者說主要用來構成這些片語。例如：
- Come at once! (馬上過來！)
- They met at my birthday party and became good friends at once. (他們在我的生日派對上相識並很快成了好朋友)
- Don’t all talk at once! (不要同時都說話！)
- He can do two things at once. (他可以同時做兩件事)
- She left all at once. (她突然離開了)
- For once Jack was telling the truth. (傑克就這一回說了實話)
- You can pay the bill just for once. (你難得一次可以付帳)
- He only came the once. (他只來過一次)
- Do it just the once. (就做這一次吧)
- Sam did it just this once, but once is enough. (山姆只做了這一次，但一次就夠了)
- We met just the/that once, but I still remember her very clearly. (我們就見了那一次，但我仍很清楚地記得她)