"开心" 在英语中可以用 "happy" 来表示。"Happy" 是一个形容词，源于古英语 "hap" 或 "hapiness"，意思是 "幸福的" 或 "高兴的"。

例句：

I'm so happy to see you. (我看到你真开心。)
She was happy with her new job. (她对新工作感到很开心。)
He looked happy when he won the prize. (他获奖时看起来很开心。)

如果你想表达某人很开心，还可以使用其他形容词，如 "depghted"（高兴的）、"thrilled"（兴奋的）、"ecstatic"（狂喜的）、"overjoyed"（非常高兴的）等。

例句：

She was depghted to receive a gift from her friend. (她收到朋友的礼物很高兴。)
He was thrilled to see his favorite band in concert. (他看到自己喜欢的乐队演唱很兴奋。)
She was ecstatic when she heard the good news. (当她听到好消息时很狂喜。)
He was overjoyed when he found out he had won the scholarship. (他得知自己获得了奖学金非常高兴。)