Assure

Verb

1. To make sure and secure; ensure.
2. To give (someone) confidence in the trustworthiness of (something). .mw-parser-output .object-usage-tag{font-style:italic}.mw-parser-output .deprecated{color:var(--wikt-palette-grey-lime-8,olivedrab)}[with that; or with of]

   "I assure you that the program will work smoothly when we demonstrate it to the client."

3. To guarantee, promise (to do something).
4. To reassure.