verb
BAD : We ensure you that we will do our best.
GOOD : We assure you that we will do our best.
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ensure = to make certain (that something will happen or exist): 'We need to ensure that our prices remain competitive.' 'Measures are being taken to ensure public safety.'
assure = tell someone that something will definitely happen or is definitely true, especially to make them feel less worried: 'I assure you that the report will be on your desk by tomorrow lunchtime at the latest.' 'The doctor assured me that there was no need for alarm.'