我愿意英语怎么说？

这个要放在语境里面去理解的啊
要是问题是Do you want to go swimming with me? Yes, I do.
Will you marry me ? Yes, I do.(请注意，虽然 I will 语法是正确的，但是为了表达强烈的意愿，一般我们使用I do更为多些~~）
Would you like a cup of tea? I'd love to...
呵呵~~希望对你有所帮助~

如果对方问你“愿意吗？”will you? would you?

直接回答：我愿意。I do!
如果直接说我愿意做什么，就是：I will do...
I'm willing to do ...
I'm ready to do...
I'd like to do ...

I do!

例如：Will you marry me? 你愿意嫁给（娶）我吗？
--Yes, I DO！ 是的，我愿意！

具体语境具体分析
1.别人提议做某项活动，如：Would you like to come to my birthday party?
你可以回答：Yes, I'd love to. 或 Yes, I'd like to.
2.用want 提问时，如Do you want to come to my birthday party?
你可以回答： Yes, I do.
3.别人求婚时,回答：Yes, I do.