我非常同意可以说成 "I completely agree"。这句话的来源是英语中 "I agree" 的表述方式。例如：

I completely agree with you. (我非常同意你的看法。)

I completely agree that we should do something about this. (我非常同意我们应该采取行动解决这件事。)

I completely agree that it's the right thing to do. (我非常同意这是一件对的事情。)

希望这些例句对你有帮助。