Isn't is the ordinary short form of is not. You push is and not together, the o in not drops out, and an apostrophe marks the missing letter: isn't. It is the form people reach for in speech, in messages, and in writing that is meant to sound like speech.

Isn't is the ordinary short form of is not isn't是is not常見的縮寫。You push is and not together把is和not黏在一起，the o in not drops out not裡的o被拿掉，and an apostrophe marks the missing letter用一個撇號標出被拿掉的字母：isn't isn't。It is the form people reach for in speech這是人們講話時會用的形式，in messages在訊息裡，and in writing that is meant to sound like speech以及故意寫得像口語的文字裡。

You also hear it at the front of a checking question: Isn't this yours? which is less a request for new facts than a way of saying I think this is yours — tell me if I am wrong.

You also hear it at the front of a checking question你也會聽到它放在確認問句的開頭：Isn't this yours?「Isn't this yours?」（這不是你的嗎？）which is less a request for new facts與其說是在問新的事實 than a way of saying I think this is yours不如說是在說「我覺得這是你的」——tell me if I am wrong如果我猜錯了告訴我。

Though actually, school English still treats the full form as the polite one on formal paper, so the same student may write is not in an exam and say isn't to a friend two minutes later. Older grammar books call the short form speaking-only; younger writers type it everywhere, even in quite careful emails.

Though actually不過事實上，school English still treats the full form as the polite one on formal paper學校英文在正式文件上還是把完整形式當作比較禮貌的寫法，so the same student may write is not in an exam所以同一個學生可能在考試裡寫is not and say isn't to a friend two minutes later兩分鐘後跟朋友講話卻說isn't。Older grammar books call the short form speaking-only比較舊的文法書把縮寫形式歸類為只用在口語；younger writers type it everywhere年輕的寫作者到處都打這個縮寫，even in quite careful emails就連寫得很仔細的email裡也是。

It still means exactly is not. It is not a softer word, and it is not a different verb. One more real-life wrinkle: with I we say I'm not, not I isn't, and in some places you will hear ain't instead — teachers mark that last one as non-standard, but you will meet it.

It still means exactly is not它的意思還是完全等於is not。It is not a softer word它不是比較委婉的說法，and it is not a different verb也不是不同的動詞。One more real-life wrinkle還有一個實際上的小狀況：with I we say I'm not用I的時候我們說I'm not，not I isn't而不是I isn't，and in some places you will hear ain't instead而且在有些地方你會聽到用ain't代替——teachers mark that last one as non-standard老師會把最後這個標成不標準，but you will meet it但你還是會遇到它。