Difference between must and have to?

[vanessa]

Test No. errors/inter-13 "Learning new words", question 7

So, let's see how we can explain the word 'customer': First we must to put it into a category. Is it food? Can you eat it? Or is it a machine or a device?

(a) let's see
(b) can explain
(c) must to
(d) device

Test No. errors/inter-13 "Learning new words", answer 7

So, let's see how we can explain the word 'customer': First we must put it into a category. Is it food? Can you eat it? Or is it a machine or a device?

Correct entry: must
The error was: (c) must to

You have found the error but your entry is incorrect.
So, let's see how we can explain the word 'customer': First we have to put it into a category. Is it food? Can you eat it? Or is it a machine or a device?
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why i can use have to in this sentence?

[Mister Micawber]

.
Have to and must can have slightly different connotations, but they are often used as synonyms, with have to less formal than must.

Many authorities consider that must indicates an internal decision of the speaker, while have to indicates the presumption of an outward authority, but in practice these often overlap so extensively that either will serve.
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Native English teacher at Mister Micawber's