The interjection 嗨
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.
The interjection 嗨 
can express different emotions:
1) joy, confidence. It can be translated as “wow” or “yeah!”.
can express different emotions:
1) joy, confidence. It can be translated as “wow” or “yeah!”.Examples
嗨，咱们工人有力量！
Yeah, us workers have strength!
hāi, zánmen gōngrén yǒu lìliàng
嗨，你姐姐真棒！
Wow, your sister is really cool!
hāi, nǐ jiějie zhēn bàng
2) annoyance, regret. Can be translated as “eh” or “oh”.
Examples
嗨，太可怜！
Eh, what a poor guy he is!
hāi, tài kělián
嗨，我现在怎么处理那么多的问题呢？
Oh, how will I solve so many problems?
hāi, wǒ xiànzài zěnme chǔlǐ nàme duō de wèntí ne
Also, 嗨 is used as a greeting, because it’s phonetically the same as the English word “Hi”.
Examples
嗨，去哪里？
Hi, where are you going?
hāi, qù nǎlǐ
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