唉
唉 can be an interjection or a sound-imitative word. Let's look at both of them.
The interjection 唉
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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 唉 has several meanings and readings:
1) 唉 
expresses agreement or readiness. It can be translated as “uh-huh!”, “okay!”.
expresses agreement or readiness. It can be translated as “uh-huh!”, “okay!”.Examples
唉，那我先走了
Okay, I'll go then
āi, nà wǒ xiān zǒu le
唉，要是他懂英语该多好啊！
Uh-huh, if only he knew English!
āi, yàoshi tā dǒng yīngyǔ gāi duō hǎo a
唉, 我知道了
Uh-huh, I got it
āi, wǒ zhīdào le
2) 唉 expresses annoyance or regret. It can be translated as “alas”.
expresses annoyance or regret. It can be translated as “alas”.Examples
唉！我的电脑又当机了
Alas, my computer crashed again!
ài! wǒ de diànnǎo yòu dāngjī le
唉！他不小心摔伤了
Alas, he fell and hurt himself!
ài! tā bù xiǎoxīn shuāishāng le
The sound-imitative word 唉唉
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Sound-imitative words are also called "onomatopoeia" (象声词).
When repeated, the onomatopoeia 唉唉 imitates the sound when someone is crying or whimpering.
imitates the sound when someone is crying or whimpering. Examples
别再唉唉叫！否则我真的让你哭出来！
Stop whimpering, or I'll really make you cry!
bié zài āiāi jiào! fǒuzé wǒ zhēnde ràng nǐ kūchulái
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