Long 用作副詞或形容詞，可用來指時間、距離和長度。然而，這個幾乎在我們一開始學英文就會學到的字，卻有一些用法可能讓人用錯而不自知。Long 的用法陷阱主要存在於它用作副詞來指時間的時候。本文將僅就此詳加說明，其他用法並無深究的必要，自是無庸贅述，或者礙於篇幅暫不予討論。
Long 用作副詞來表示時間「很久」時，只能用在疑問句和否定句。例如：
- How long have you been here in Taipei? (你來台北多久了？)
- Will you be long, or shall I wait? (你要很長的時間嗎，我要不要等你？) － “Be/take long” 意為「要很久；要很長的時間」。
- Don’t be long getting ready, please. (請馬上準備好)
- I hope you haven’t been waiting long. (我希望你沒有等很久)
- John didn’t stay long in the library. (約翰沒有在圖書館待很久)
Long 單獨一個字不可用在肯定句。我們通常使用 (for) a long time。例如：
- John stayed for a long time in the library. (約翰在圖書館待了很久) (正)
- John stayed long in the library. (誤)
- It took a long time for me to write this article. (寫這篇文章花了我很長的時間) (正)
- It took long for me to write this article. (誤)
- A: Will it take long? (A：那要很長的時間嗎？) (正)
B: No, it won’t take long. (B：不，不要很長的時間) (正)
或
B：Yes, it’ll take a long time. (B：是的，那要很長的時間) (正)
B：Yes, it’ll take long. (誤)
不過，當 long 與 too, enough 和 so 連用時可單獨用在肯定句。例如：
- Two weeks is too long to wait for an interview. (等候面試要等兩個禮拜太久了)
- Mary stayed just long enough to meet her boyfriend’s parents. (瑪麗只待到和她男友的父母見了面)
- His father waited long enough for an appointment. (他父親等候任命等得夠久了)
- Cindy asked her boyfriend why he took so long to call her. (辛蒂問她男友怎麼這麼久才打電話給她)
- What took you so long? (你怎麼這麼久？)
在談到實質的時間長度時，我們可以在肯定句 (疑問句和否定句當然也可以) 使用「時間 + long」以及 all day/night/week/month/year long (整天 / 整晚 / 整週 / 整月 / 整年) 和 my whole life long (我的一輩子) 等片語 (long 在這些片語中都是副詞)。例如：
- The concert was four hours long. (演唱會長達四小時)
- They worked all day long. (他們整天都在工作)
我們亦可在肯定句中使用 a long time ago (很久以前)、long ago (很久以前)、long before (在…之前很久)、long after (在…之後很久) 和 long since (很久以前)。例如：
- The hospital was established a long time ago. (這家醫院建於很久以前)
= The hospital was established long ago. - You should have ended the extramarital affair long ago. (你早該結束這段婚外情)
- Kevin knew his wife long before they got married. (凱文在結婚之前很久就認識他太太了)
- Allen left not long before you arrived. (你抵達時艾倫剛離開不久)
- Jack was born long after his parents got married. (傑克的父母結婚多年後才生下他)
- They’ve long since gone home. (他們早就回家了)
- I learned my lesson long since. (我早就學到教訓了)