The interjection 哎
An interjection (感叹词) is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction. It expresses feelings and emotions but does not name them. It is usually placed at the beginning of a sentence.
哎 
can express different emotions:
1) fright, pain, impatience, regret, annoyance, reproach. It can be translated as "ah", "eh".
can express different emotions:
1) fright, pain, impatience, regret, annoyance, reproach. It can be translated as "ah", "eh".Examples
哎! 太丢脸呢！
Ah, what a shame!
āi! tài diūliǎn ne
哎! 您怎么能那样说呢？
Eh, how can you say that!
āi! nǐ zěnme néng nàyàng shuō ne
2) caution, while drawing attention to something. It can be translated as "hey!".
Examples
哎，你看那人，真有意思！
Hey, look at that man. How interesting!
āi, nǐ kàn nà rén, zhēn yǒuyìsi
哎，小心点儿！
Hey, be careful!
āi, xiǎoxīn diǎnr
3) amazement, surprise. It can be translated as "oh!".
Examples
哎，你怎么来了？
Oh! Why are you here?
āi, nǐ zěnme lái le
哎，他买新车了！
Oh, he bought a new car!
āi, tā mǎi xīn chē le
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