乐于 用英语怎么说

"乐于" 用英语可以说成 "be willing to" 或 "be happy to"。
"Willing" 意思是 "愿意的"，"happy" 意思是 "高兴的"，所以 "be willing to" 和 "be happy to" 均可表示 "乐于" 的意思。
例句:
I'm willing to help you with your project. (我乐意帮助你的项目。)
She's happy to lend a hand. (她乐意提供帮助。)
They are willing to invest in the new business. (他们乐意投资新公司。)
He's happy to teach you how to play the guitar. (他乐意教你怎么弹吉他。)
来源：Will 和 Happy 是英语中常用的形容词，在句子中与动词连用表示愿意或高兴的意思。