1. 用作副詞，表示“一次”，是頻度狀語，通常放在句末，有時(shí)也放在句首。如：
I’ll help you once but then you must do it by yourself. 我只幫你一次，但以后你就得自己做了。
Once he made bold to ask her to accompany him to a party. 有一次他鼓起勇氣要求她陪他去參加一個(gè)晚會(huì)。
表示“曾經(jīng)”“一度”，是表示不確定時(shí)間的副詞，通常放在行為動(dòng)詞之前，特殊動(dòng)詞（助動(dòng)詞、情態(tài)動(dòng)詞、動(dòng)詞 be）之后，有時(shí)也可放在句首。如：
Once there stood an old church on the hill. 山丘上曾經(jīng)有一座古教堂。
This novel was once very popular but nobody reads it today. 這本小說曾經(jīng)很受歡迎，但現(xiàn)在沒有人讀了。
表示“一次”的once還經(jīng)常與 a, every 等連用。如：
This magazine comes out once a week. 這份雜志每周出一次。
The earth turns on its axis once every 24 hours. 地球每24小時(shí)自轉(zhuǎn)一圈。
2. 用作連詞，引導(dǎo)時(shí)間狀語從句，注意從句要用現(xiàn)在時(shí)表示將來意義。如：
Once you let him start talking, there was no stopping him. 一旦讓他開始講話，就沒法止住他。
Once you have learned Spanish, you will find Italian easy. 一旦你學(xué)會(huì)西班牙語，你會(huì)覺得意大利語容易。
所引導(dǎo)從句的.主語若與主句主語一致，有時(shí)也可用其省略結(jié)構(gòu)，被省略部分通常（并不總是）包括有動(dòng)詞 be。如：
Once (it is) printed, the book will be very popular. 一旦出版，這本書會(huì)大受歡迎。
Once having made a promise, you should keep it. 你一旦許下諾言，就該遵守。
Once (he was) in the examination hall, he forgot about all this. 他一進(jìn)考場(chǎng)，這一切他都忘了。
3. 比較 at once 與 all at once：前者表示“馬上”，后者表示“突然”。如：
If you don’t start at once you’ll be late. 除非你立即動(dòng)身，否則你就會(huì)遲到。
All at once, we heard a loud noise. 猛然間我們聽到了一聲巨響。
另外，兩者都可表示“同時(shí)”“一起”。比較：
I can’t do two things at once. 我不能同時(shí)做兩件事。
Everybody started speaking all at once. 大家同時(shí)開始說起來。
once的用法例句
1. It had once been the home of a wealthy nobleman.
這里曾是一個(gè)有錢貴族的宅邸。
2. We tend to meet up for lunch once a week.
我們往往每周共進(jìn)一次午餐。
3. He once told an interviewer that he didn't even like rock music.
他曾告訴一位采訪者，他甚至都不喜歡搖滾樂。
4. Once wholesale prices are deregulated, consumer prices will also rise.
一旦批發(fā)價(jià)格放開，零售價(jià)格也會(huì)隨之上漲。
5. He took off at once and headed back to the motel.
他立刻離開，回到汽車旅館。
6. The car passed over the body twice, once backward and then forward.
那輛車把那人來回軋了兩次，先是倒著軋的，再是往前開著軋的。