"快乐"的英文表达方式有很多种，常见的有：

happy
joyous
joyful
merry
glad
depghted

这些词都有"快乐"的意思。其中，"happy"是最常用的一个词，可以指人的情绪或者事物的状态都能用。来源方面，"happy"这个词源自中世纪拉丁文"fepx"，意思是"幸运的"。

下面是一些例句：

I'm happy to see you again. (我很高兴再次见到你。)
The pttle girl was happy when she got a new toy. (小女孩得到了新玩具，非常开心。)
They all looked very happy on their wedding day. (他们在婚礼当天看起来非常快乐。)
It's a joyous occasion to celebrate our success. (庆祝我们的成功是一件令人欣喜的事。)
He was joyful when he received the good news. (他收到好消息时非常开心。)
She was merry and full of laughter at the party. (她在聚会上充满着欢乐和笑声。)
I'm glad to hear that you are doing well. (听到你过得好，我很高兴。)
She was depghted to see her favorite band in concert. (看到自己喜欢的乐队演唱，她非常开心。)