As your bodyguard, I'm here to ensure your safety.
✗ As your bodyguard, I'm here to assure your safety.
I know you're worried, but let me assure you that you're safe with me.
✗ I know you're worried, but let me ensure you that you're safe with me.
The word "ensure" means "to make certain".
Please ensure that you remember to bring your passport.
It's very important to bring your passport, so make sure not to forget it!
You assure someone when you make them feel confident in something and dispel any doubts.
I was worried about earthquakes, but Sally assured me that they're vary rare.
Sally convinced the speaker that earthquakes aren't a big concern.
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