我保证用英语怎么说？

我保证: by my troth

Examples:
1. 他向我保证星期五一定准备好.
He gave me an assurance that it would be ready by Friday.
2. 我保证我没有告诉过任何人。
I didn't tell anyone, on my oath.
3. 不客气。我保证您会发现我们产品的前景非常美好。
My pleasure. I can assure you that you'll find very good prospects in our products.

I assure ...

i ensure that

i promise 或者说 i dare say

i promise i bet