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kāi xīn
adj. happy
Discussion of 开心/開心
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In this post, we discuss these four adjectives: 愉快, 高兴, 开心 and 快乐. They all mean “happy”. What are some of the important differences?

1.高兴 modifies one’s mood/ frame of mind (how the person feels inside) as well as describes the person’s external state (facial expressions, body languages)
快乐 and 愉快 are more about the person’s inner feelings.
开心 is more about the person’s facial expressions and body languages.
We use 高兴 and 开心 on people, but not on events or experiences; but we can use 愉快 and 快乐 to describe an event or experience. E.g. 快乐的结局, 愉快的经历

2. We can use reduplicated forms of 高兴, 快乐 and 开心. Example: 高高兴兴、快快乐乐、开开心心. But 愉快 cannot be used in this form.

3.愉快 and 快乐 have the same meaning, but 愉快 is more formal, while 快乐 is used both in formal and informal situations. in addition, when we wish someone a happy time when it’s a holiday, his or her birthday, etc., we usually only use 快乐. E.g. 生日快乐!新年快乐!周末快乐!Rarely do we use 愉快, however, using 愉快 in the examples above is grammatically and synthatically correct. It’s just we have grown so used to using 快乐 in these contexts. Just one reminder: you might hear 周末愉快 from time to time, and it is appropriate to say this.
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