哈
哈 can be a sound-imitative word or an interjection. Let's look at both of them.
The sound imitation 哈
Pay attention
Sound-imitative words are also called "onomatopoeia" (象声词).
哈 
is a sound imitation of:
1) a laugh. In this case, it’s usually used with repetition: “ha-ha”
is a sound imitation of:
1) a laugh. In this case, it’s usually used with repetition: “ha-ha”Examples
哈哈，太搞笑！
Haha, how funny!
hāhā, tài gǎoxiào
我只说了一句话之后，他突然哈哈大笑起来
I only said one sentence and he suddenly laughed out loud
wǒ zhǐ shuō le yí jù huà zhīhòu, tā túrán hāhā dà xiàoqilái
2) an exhalation: “whew”, “phew”.
Examples
哈，考试结束了，可以休息了
Phew, exams are over. Now we can rest!
hā, kǎoshì jiéshù le, kěyǐ xiūxi le
The interjection 哈
Pay attention
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 哈 expresses:
1) satisfaction or delight, usually used with repetition.
expresses:
1) satisfaction or delight, usually used with repetition.Examples
哈哈，我猜到了！
Haha, I guessed it!
hāhā, wǒ cāidào le
哈哈，我成功了！
Haha, I got it!
hāhā, wǒ chénggōng le
2) Exclamation or loud shouting. It can be translated as “aha”, “there you go!”.
Examples
哈，我就知道是你!
Aha! I knew it was you!
hā, wǒ jiù zhīdào shì nǐ
哈，你不会跑了！
Aha, you can't escape!
hā, nǐ bú huì pǎo le
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