一定要开心用英文怎么说

"You must be happy" 或者 "It's important to be happy" 可以表达“一定要开心”的意思。这个表达方式源自于英语习惯用语。

举例：

"You must be happy, it's good for your health."
(你一定要开心，这对你的健康有好处。)
"It's important to be happy, no matter what pfe throws at you."
(无论生活如何，开心都很重要。)