- 1不該、不應該（should not 的縮寫）
Shouldn't is the everyday short form of should not. You use it to give advice (You shouldn't skip breakfast), to say what is not right (People shouldn't be cruel), or to say what you do not expect (She shouldn't be long = I think she will not take long). It is not a softer or ruder should not: same meaning, shorter shape. In speech, messages, and almost all real writing, shouldn't is the normal word. In very careful school essays and formal letters, some teachers still want should not written out in full — though actually even news and books use the short form all the time. The n't is the same little piece as in don't, can't, and wouldn't: it is not squeezed onto the end. Some people talk about the shoulds and shouldn'ts, meaning the pile of rules about what you ought and ought not to do.
「shouldn't」是 should not 的日常縮寫，意思一樣，只是寫法較短，不是比較溫和或比較粗魯的說法。它常用來給建議，例如「You shouldn't skip breakfast」；也用來說什麼不對，例如「People shouldn't be cruel」；還可以表示推測，例如「She shouldn't be long」，意思是我想她不會花很久。
在口語、訊息和幾乎所有實際寫作中，「shouldn't」都是正常用法。只有在很講究的學校作文和正式信件裡，有些老師還會要求完整寫出 should not；不過新聞和書籍其實也常用縮寫。它的 n't 和 don't、can't、wouldn't 的 n't 是同一個小部件。
From the word’s history
should + -n't.
由 should 加上 -n't（not）組成。
組成From should + -n't (-n't is a short form of not).
原始詞典資料2 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
shouldn'tverb/ˈʃʊdənt/
- 1
Should not (negative auxiliary)
- I shouldn't be telling you all this. It'll make you angry.
shouldn'tnoun/ˈʃʊdənt/
- 1
informalSomething that should not be done.
- Being a list-o-maniac, I suggested we make a list of the "shoulds" and "shouldn'ts." So in the darkness of hazy sleep, I began to mentally prepare mine. The first item on the "should" side was easy: a sibling for our 3-year-old daughter.
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。