Must / Have to (+ and -)

Look at that snow!! It must be very cold outside!

Look at that snow!! It has to be very cold outside!

You must eat healthier so your health can improve

You have to eat healthier so your health can improve

I have to visit my grandma _{(external obligation, your grandma requested you to do that)}

I must visit my grandma _{(personal obligation, the speaker thinks it is necessary)}

I have to do an essay about pollution, so I can't go out

The students have to take the San Diego test to prove they have a B1 level to graduate.

My sister has to wear formal clothes for her new job