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.