Must or have to

[Rachel Adams]

If something is important to me I use "must", if I use "have to" it means someone else thinks it's important, but if I am not talking about myself which modal verb should I use "must", "have to", or "should"?

"I think she has to/must/should stop smoking."

 

[GoesStation]

Should, if you're expressing your opinion. Has to if you're expressing your understanding of a medical necessity. We hardly ever use must in this context in American English.

 

[Rachel Adams]

Should, if you're expressing your opinion. Has to if you're expressing your understanding of a medical necessity. We hardly ever use must in this context in American English.

Do you mean the obligation comes from someone else (a doctor) with "have"? But in this case I wouldn't be expressing my understanding. Because the books I have explain it in a different way. This example or something similar isn't used in either of them. You said "if you are expressing your understanding" do you mean " you are the one who thinks it's important"?
This is the explanation I got used to: "With must the obligation comes from the speaker. To talk about an obligation that comes from "outside" we usually prefer have to."

1. "I must stop smoking." I want to.

2. "I have to stop smoking." Doctor's orders.

At the same time the book says "Have to can also be used to talk about obligation coming from the speaker or the hearer, in the same way as must. This is normal in American English which uses must less often in this sense and is becoming very common in British English."

1. "I really have to stop smoking." (Or I must)

2. "Do I have to clean all the rooms?" (Or must I?)

But who is "the hearer"?
It's from Michael Swan's "Practical English Usage"

 

[Yankee]

Do you mean the obligation comes from someone else (a doctor) with "have"? But in this case I wouldn't be expressing my understanding. Because the books I have explain it in a different way. This example or something similar isn't used in either of them. You said "if you are expressing your understanding" do you mean " you are the one who thinks it's important"?
This is the explanation I got used to: "With must the obligation comes from the speaker. To talk about an obligation that comes from "outside" we usually prefer have to."

1. "I must stop smoking." I want to, but I haven't made the commitment

2. "I have to stop smoking." Doctor's orders. Perhaps, but could also be my desire.

Some other possibilities.