Boss: "Please ensure that you finish the job by Friday. Employee: I can assure you that I will finish the job by Friday". When you "ensure that" something will happen, you take action to make it happen. When you "assure someone" that something will happen, you promise then that it will happen.

ASSURE: to promise or say with confidence (it is more about saying than it is about doing). ENSURE: to do or have what is necessary for success.

You can also "reassure" someone, which is a bit like assuring someone, but more about putting them at ease. "Can you reassure Peter that everything will be ok?" The point is, "assure" and "reassure" are usually things you do to people rather than things. "Ensure" means "make sure", and is not typically something you do to a person. In Michael's example, "Please ensure that you finish the job", the "you" follows "ensure" (it could also be "Please ensure you finish the job"), but "you" is not the direct object. I hope that makes sense.