A short lesson today, focusing on the difference between insure, ensure and assure. Let me start by saying that some sources insist there is no difference between insure and ensure. Most, however, agree with me that in standard American English there is a huge difference. If you follow these guidelines, no one will ever say you're wrong. Why take the chance?
To insure means to guarantee payment if something happens. Money nearly always changes hands. If you're actually buying insurance, you are insuring something: He insured the old jalopy for much more than I thought it was worth.
If you are acting to make sure something happens, you are ensuring that it will: If you can substitute be sure or make sure, the word you're looking for is ensure. I will put up a gate to ensure my grandson doesn't fall down the steps.
If you are trying to instill confidence in a person or animal, you will assure them. Assure applies to a living being. In fact, it's nearly always followed by a noun referring to a person (you, him, her, etc.): I assured him that his child would be safe near the steps.
Remember: Insure -- money changes hands. Assure relates to something alive. Ensure is to be sure or to make sure.