Assure is something you do to a person, a group of people, or an animal to remove doubt or anxiety, as in Squiggly assured Aardvark that he'd come to the party early. You can remember that assure can only be used with things that are alive (and both assure and alive start with a). Only things that are alive can feel doubt or anxiety, so only they can be assured. Ensure is something you do to guarantee an event or condition, as in To ensure there'd be enough food, Aardvark ordered twice as much food as last year. You can remember that guarantee has those two e's on the end to help you remember that to ensure (with an e) is to guarantee something. Insure can be done to a person, place, or thing, but it's reserved for limiting financial liability, most commonly by obtaining an insurance policy, as in Aardvark wondered if the caterers were insured against loss. You can remember that we take out insurance to protect our income if we become unemployed, disabled, or injured in an accident. Both insure and income begin with -in

Ensure is best used to describe 'confirming an action' - e.g. Ensure the car is clean before you return it. It might best be described as meaning 'make sure'. Assure is best used to describe providing 'assurance', promise or agreement - e.g. I assure you this is perfectly safe.

Very good question. Here are two examples: The manager instructed all staff to ensure that their personal Facebook would not contain any company information, for fear of leaking commercial secrets. The staff assured the manager that they would take his instructions seriously and be ever vigilant. Ensure =  (Take some action or precautions) to make certain that (something) will (or will not) happen. Assure = tell (someone) something positive to dispel any doubts. The only possibility of confusion lies in something like "Victory is assured", or "Their army's superior strength assured them of victory." Here "assure" also means "make certain". I think a good trick is to remember these two examples of "exception" and tell yourself that in all other cases "assure" involves "TELLING someone something positive".