Assure means to say or write a guarantee. Some synonyms are: guarantee or promise
I assure you that I am not a thief!
or
I guarantee you that I am not a thief!
or
I promise you that I am not a thief!
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Ensure means to do something to make sure that something else happens
I feed my dog to ensure it doesn't die.
or
Don't walk alone at night to ensure you don't get robbed.
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native english speaker