The word "assure" is correct and can be used in both written and spoken English. It is used for when you want to tell somebody that something is certain to happen. For example, "I assure you that I will be there on time.".
"Let us assure you that the Government will do all we can to defeat the voices of division, but ultimately the challenges of integration and radicalisation cannot be solved from Whitehall alone.
"Let me assure you that my government is committed to working closely with you and your government during your tenure and most importantly, in pursuit of our bilateral relations.
"Let me assure you the situation is under control.
And, I assure you, these reports did not surface on April Fools' Day.
I can assure you that twerking does what it says on the label: there is a lot of squatting, popping, and jiggling.
Many of the IG's recommendations to assure such actions do not occur in the future have already been acted upon," he said in the statement.
Asked to describe the position of Charlie Hebdo in France today, as well as its heritage in French satire, Biard joked: "I must assure you that we don't eat children, and we don't eat believers either".
Thanks to Ludwig my first paper got accepted! The editor wrote me that my manuscript was well-written
Listya Utami K.
PhD Student in Biology, Bandung Institute of Technology, Indonesia