Soon 意為「不久」、「很快地」。就「不久」的意思而言，soon 可以指現在之後的不久 (亦即未來或過去的未來) 以及過去某個時間點之後的不久。與其他許多短副詞一樣，soon 可用在句首、句中和句末，但在指過去某個時間點之後不久的句子中，soon 不可用在句末。例如：
- The winter is coming. Soon it will be Christmas. (冬天來了。耶誕節很快就要到了) (句首位置)
- I soon realized the mistake. (我很快就意識到了錯誤) (句中位置)
- Peter will soon be here./Peter will be here soon. (彼得很快就到) (句中或句末位置，指未來)
- Mom phoned to say she’d be home soon. (媽媽打電話說她很快就回家) (句末位置，指過去的未來)
- A: Bye for now! (A：再見！)
B: Bye. See you soon! (B：再見。一會兒見！) (句末位置，指未來) - Soon all the Mayday concert tickets were sold out. (五月天演唱會門票瞬間秒殺) (正)
- All the Mayday concert tickets were soon sold out. (正)
- All the Mayday concert tickets were sold out soon. (誤)
我們可以使用 pretty、quite 或 very 來修飾 soon，用法與上述相同。例如：
- The novel will be published quite/very soon. (這本小說很快就會出版)
- If Henry doesn’t show up pretty soon, I’m leaving. (如果亨利不能快點來，我就要走了)
就「很快」的意思而言，soon 的用法與 quickly 並不相同，兩者不能劃上等號。例如：
- How soon (= when) can we have dinner? (我們什麼時候能吃晚餐？/我們多快能吃晚餐？) (正)
- How quickly can we have dinner? (誤)
- How soon can John get here? (約翰多快能趕到這兒？) (正)
- How quickly can John get here? (誤)
- I hope to see you soon. (我希望能很快見到你) (正)
- I hope to see you quickly. (誤)
Soon 可以用在 after 或 afterwards 的前面來表示「不久之後」的意思，但副詞 afterwards 的後面不可接子句或受詞。例如：
- Tom supposedly showed up at the Christmas party soon after I left. (湯姆大概是在我離開耶誕晚會後不久現身的) － 不可用 afterwards。
- They arrived soon after us. (我們到了之後不久他們就到了) － 不可用 afterwards。
- Soon after agreeing to go out to eat with George, Mary realized she’d made a mistake. (同意跟喬治一起出去吃飯之後不久，她就意識到自己犯了一個錯誤) － 不可用 afterwards。
- Teresa left first, and Emily left soon after/afterwards. (泰瑞莎先離開，不久之後艾茉莉也離開了)
As soon as … 是個相當常用的片語，其後可接形容詞 possible (as soon as possible 意為「盡快」) 或子句 (as soon as + 子句意為「一…就…」)。例如：
- Teddy wants you to call him back as soon as possible. (泰迪希望你盡快給他回電)
- Come and see me as soon as you can. (盡快來看我)
- I’ll tell him as soon as I see him. (我一見到他就告訴他)
- As soon as I saw the babysitter, I knew there was something wrong with my baby daughter. (我一看到臨時保母就知道我小女兒出事了)