assure/ensure/make sure

ASSURE: 

to tell someone confidently that something is true, especially so that they do not worry: (similar to "promise")

• https://dictionary.cambridge.org/dictionary/english/assure?q=ASSURE

• https://www.oxfordlearnersdictionaries.com/definition/english/assure?q=assure 

• He assured me I would get to the train station in time. 

• We've been assured by our bosses that nobody is going to be made redundant. 

• Bank officials assured her that employees' paychecks would be processed as usual. 

• At no time, he assured MPs, did Mr Werritty have access to classified documents.

ENSURE: to make something certain to happen:

Cambridge dictionary

Oxford dictionary

• Ensure you have firewalls on your computer, and anti-virus software and spyware. 

• It is worth booking a room early to ensure you get one of the nicer rooms. 

• You should drink two and-a-half litres of fluid a day to ensure good hydration. 

MAKE SURE VS ENSURE: 

You make sure or you ensure that something happens. 

• I'll make sure he calls you back. 

• If you're using a fireplace, make sure someone is tending the fire at all times. 

• If your employer invests in the pension on your behalf, make sure you sign up.

In these cases, 'ensure' is more formal than 'make sure'. 

If you are going to use just a noun, you can only use 'ensure': 

• Legislation to ensure equal opportunities for everyone. 

• The airline is taking steps to ensure the safety on its aircraft.