Must and have to with you

[Rachel Adams]

I read in my textbook by Michael Vince in "English Grammar in Context" "You must be careful" is a speaker's personal opinion" while "You have to be careful" an outside opinion or rule" but with the first person there is little difference sometimes between "I have to go" and "I must go". So either can be used. But according to the book there is a difference between them when "have to" and "must" are used with "you". Does Michael Vince mean that "must" with "you" means it is important for the speaker but with "have to" it is an outside rule? What's the exact difference between them when "must" and "have to" are used with "you"?

 

[5jj]

There is no 'exact difference'. Some people feel that there is no difference, some, especially in AmE, rarely use 'must', and some, like Vance feel this difference. I, like Vance, use 'must' when I am imposing my own obligation, 'have to' when I am referring to an external obligation.

Parent to child as he is leaving home:

1. You must put your tie on. (I, the parent, am ordering this)
2. You have to put your tie on. (It's a school rule.)

 

[Tdol]

Think of such distinctions as patterns and tendencies rather than absolutes, and often not followed by everybody. Like 5jj, I follow this pattern, and following it is fine.