Most writers use these words interchangeably.
But there are some differences. For example, assure is used only in reference to people. You assure your boss. If you insure him, then you’re taking out a policy on his life.
Also, insure is typically reserved for financial meanings.
It’s probably best to follows these rules:
1. Ensure an event or a result. 2. Assure a person. 3. Insure a financial risk.
Please note: If a that clause follows the verb, you must use ensure.
We want to ensure that we will win.