1. What is an interjection?
　　
　　An interjection is used as an exclamation or in greeting, as in English, e.g.
　　
　　“啊” expressing surprise or admiration, e.g. 啊，伟大的长城！(O magnificent Great Wall!)
　　“哎” is used to convey surprise or dissatisfaction, e.g. 哎，怎么不早说呢？(But why didn't you tell me sooner?)
　　“哎呀” is used to express surprise or dissatisfaction, e.g. 哎呀!我们有二十年没见面了.(Well, it's twenty years since we met last.)
　　“哎哟” is used to show astonishment or pain, e.g. 哎哟!好疼! (Ouch! It hurts!)
　　“喂” is used in greeting, e.g. 喂,你有什么事吗? (Hello, what can I do for you?)  

 
2. What is noteworthy about Chinese interjections 
 
2.1. An interjection is always used alone 
 
　　Unlike other function words, an interjection is always used alone, separately from the structure of sentence by a comma. 
 
2.2. An interjection is always used at the beginning of a sentence 
 
　　An interjection is always used at the beginning of a sentence, so we can easily differentiate an interjection and a mood auxiliary, which is always at the end or at the middle of sentence, when both the words take the same form, e.g.
 
　　啊！景色真美。(Oh, lovely view!) --- The “啊” is an interjection.
　　景色真美啊！(What a lovely view!) --- The “啊” is a mood auxiliary.