It's ensure, not assure

In my copy editing/proofreading gigs, and even in other jobs that don't primarily focus on text, people often use "assure" when they really should use "ensure." For instance, I've seen people mistakenly write "They used that instrument to assure they would be accurate." That is not correct because "assure" means, according to the Cambridge dictionary, to "promise" or "make certain" or "say with certainty...that something is true," as in "I assure you that you will get the job" or "The mayor assured the people that corruption will be investigated during her administration." Basically, it's a way to let other people know that they shouldn't worry about something. I assure you that what I'm writing is true.

"Ensure" simply means to make sure of something. So you ensure that you have your bus pass. You ensure that all the doors are locked. You ensure that you've done all the necessary paperwork. Basically, when you're thinking "I want to make sure," use "ensure."

Here's a visual: the name of the drink Ensure implies that you want to make sure, i.e., ensure, that you get all the nutrients you need.