You have my word.我向你保证。

I give you my word.我向你保证。

意思类似 I promise. /I promise you.

区别：

You have my word 类似中文 “大丈夫一言既出，驷马难追”，表达的感情更强烈，体现的是人信守承诺的品格。

promise大概就上升不到“人格”的高度。

He is a man of his word. 他是个信守诺言的人。

He is not a man of his word.他这人不讲信用。

His word means nothing.他说话不算话；说话跟放屁一样。