Mostly the same, but they’re used slightly differently. 高兴 generally isn’t used to describe long-term happiness (if I wanted to translate the phrase “a happy childhood”, I would probably say 快乐童年). I would use 高兴，or maybe 开心，though, to describe how I feel after getting a good score on an exam, which is a more fleeting sort of pleasure. 快乐 is often more long-term， although i dunno if it’s exactly /wrong/ to use it to describe shorter periods of time. It’s a pattern, but not necessarily a rule. The biggest different is that in phrases like “happy new year” or “happy birthday” you have to use 快乐 or sometimes 愉快。 for example, 圣诞节快乐，生日快乐

高兴： 1）愉快而兴奋,表现在一种即时状态；一般多用于口语。 Happy and excited, showing in a state of instant; usually used in spoken language. 看到你，我心里真高兴。 在初次与人见面时，常说“认识你很高兴！” 2）“高兴”是形容词也是动词，“高兴”做动词时，意思是“喜欢做某事，对……感兴趣，多用于否定式。 Adjectives are also verbs，When doing a verb, it means "like to do something, to..." Be interested。More used in the negative 路太远，我们不高兴去. 快乐： 感到幸福或满意,表一段时期.；一般多用于书面语；没有动词词性。 Feel happy or satisfied, for a period of time; generally used more in written language.，can’t be used as verb. Fixed phrase：祝你生日快乐！新年快乐！

高兴，内心此刻的一种情绪表达，是因为外界的影响而产生的，是一时的；而快乐是从心底涌出来的兴奋，是长久的。 你可以这样理解。一个人开心并不代表快乐，也许乐极生悲。快乐指长时间心情都很好。

Hi, Drake I think in most situation, they are the same. Both of them can be used to express one's happiness. But there is a special sotuation you can only use "快乐" rather than "高兴"--when you are writing or speaking a blessing to others. Like "新年快乐" - Happy New Year "生日快乐" - Happy Birthday We can say "新年高兴" or "生日高兴", it is too weird. Just like we cannot say "Cheerful New Year" or "Cheerful Birthday", right? What are the difference between "Happy" and "Cheerful"? I think it is a tricky question, lol. Feel me to ask me if you have any questions. PS: Could u help me correct my mistakes if there are any. Meow~ (- v-)~

基本上是一样的，都是happy！非要区分的话，高兴多用在口语，且多和人搭配，比如今天真高兴！你高兴吗？开心稍微书面，且用于物，例如一只开心的小猫，一张开心的笑脸。