介係詞對我們來說有點難度，因為我們用中文講到時間或地點時，並不需要用各種介係詞帶出來該時間或地點，所以使用英文時，對於要用哪個"在"(in, on, at)很困惑，我們就一起看看下面說明。
[時間]
on a DAY. in a PERIOD of time. at a SPECIFIC TIME.
大致上來說，
on接日子，例如：
on Sunday/ Monday/ Tuesday...
on October 27th/ my birthday...
* on Christmas Day/ Easter Sunday/ Dragon Boat Festival...
in接一段時間，例如：
in January/February/ March...(月份)
in (the) spring/ summer...(季節)
in 2023/ the 2020s...(年)
in the 21st century/ the next century...(世紀)
in the Ice Age/ the Middle Ages...(時期)
in the past/ the future
at接一個精確時間，例如：
at 7 o'clock/ 3:15...
at noon/ midnight
at bedtime/ dinnertime/ the moment...
at sunrise/ sunset
*at Christmas/ New Year...
注意* 您可能發現，為什麼一樣"聖誕節"有on也有at？因為on Christmas Day指12月25日那天，on Christmas Eve指12月24日那天，而at Christmas指聖誕假期(在國外通常是12月24日到1月初，有一首聖誕歌The Twelve Days of Christmas，可想而知那幾天就是聖誕假期)，所以要敘述某事發生在聖誕假期期間，就要用at Christmas，一樣的概念適用於其他節日，所以at節日假期期間。
另外，特別留意一天內的時間介係詞 in the morning/ afternoon/ evening 但是 at night/ noon/ midnight，noon(中午12點) midnight(半夜12點)是一個精確時間，用at沒問題，但是night(晚上)不是一段時間嗎？為什麼不能像 morning(早上) afternoon(下午) evening(傍晚到晚上) 一樣用in呢？讓我們這樣想吧~ night 和 evening 相比，的確是相對較短時間，就用at短時間、in長時間來區分。
如果，星期X的早上/下午/晚上呢？用on：on Sunday morning / on Saturday evening(s)...
最後，一個小小注意事項，遇到時間前面有 last, next, every, this，不用介係詞：this afternoon / last April / every Wednesday...
最後的最後😅over the weekend = during the weekend = on/at the weekend，美式 on the weekend = 英式 at the weekend，美式 on (the) weekends = 英式 at weekends
小試身手(請填入 in / on / at / X)
1. His birthday is ___ August 21st.
2. The moon was bright ___ last night.
3. They finished the test ___ the same time.
4. I sometimes work ___ the weekend.
5. Let's meet up for a cup of coffee ___ next weekend.
6. I'll be back ___ half an hour.
7. He's not available ___ the moment. Can you call back ___ this afternoon?
8. She always visits her relatives ___ Chinese New Year.
9. I met him ___ 1993.
10. I will have a meeting ___ next Tuesday morning.
(解答 1. on 2. X 3. at 4. on/at 5. X 6. in 7. at, X 8. at 9. in 10. X)
[地點]
at a certain POINT. in an ENCLOSED space. on a SURFACE.
帶出地點的介係詞還有其他幾個，以後另闢文章說明，這裡先介紹這三個較易混淆的。
大致上概念，
at接一個明確(地)點，例如：
at the corner/ the entrance/ the door/ the bus stop/ the movie theater/ 地址...
in接一個封閉空間(有範圍限制)，例如：
in the garden/ a building/ Taipei/ Taiwan...
on接觸表面，例如：
on the wall/ the ceiling/ the door/ a page/ the floor...
注意：在交通工具方面，坐進去就不能站起身走動，用 in；可以站起身走動，或是"騎在上面"，用 on
(in a car/ a taxi/ a helicopter/ a boat ；on a bus/ a train/ a plane/ a ship/ a bike/ a motorcycle/ a horse)
如果不想太傷腦筋去判斷，可以多背一些常用語:
at home/ school/ university/ college/ work
at the top/ the bottom/ the side/ the end
in the lift(elevator)/ a row/ the sky/ the newspaper
on TV/ the radio/ the left/ the right/ the way/ the menu/ a page
至於是 in或on the street呢？通常英式用 in the street，美式用on the street。雖然也有人認為意思有點不同，但差異不大，不同地區有不同慣用與解讀，所以我們就不深究。
那麼，"在樹上"是 in the tree 或 on the tree? 如果是自己可移動的，如鳥、松鼠、昆蟲等，in the tree，因為樹有枝幹樹葉等，形成一個封閉空間，這些生物在這樹的空間裡；如果是樹的一部分，如花、葉、果實，on the tree，它們都是長在樹枝表面，所以on就很合理。
小試身手(請填入 in / on / at)
1. When did you arrive ___ the office?
2. The library is ___ the end of the street.
3. He wrote his name ___ the cover of the book.
4. You can visit me ___ 165 Zhongxiao East Road ___ Taipei.
5. Let's get ___ the car!
6. There are some beautiful pictures ___ the wall ___ our classroom.
7. We live ___ the second floor.
8. I'm sitting ___ my desk ,and my laptop is ___ my desk.
9. Sally usually meets her friends ___ school.
10. Look! There's an owl ___ the tree.
(解答 1. at 2. at 3. on 4. at, in 5. in 6. on, in 7. on 8. at, on 9. at 10. in)