Modal verbs

I won't deal with the internal or external aspects of must and have to which some native speakers see and others don't. I would like to point out a difference on which, hopefully, all agree. When referring to the past, there is a distinct difference in usage.

He has had to buy a new coat.
- expresses obligation

He must have bought a new coat.
- does not express obligation but inference

Must + a perfect infinitive never expresses obligation.

Cheers
CB

Cool Breeze wrote:

He has had to buy a new coat. - expresses obligation It could be a need, not an obligation. He must have bought a new coat. - does not express obligation but inference Yes, thinking that something had taken place with a certain probability. Must + a perfect infinitive never expresses obligation.

To my beliefs, "must" is a lot like "have to" in a way that they both mean one is required to do something. However, "must" has a more sense of urgency to it.

e.g. "I have to do my homework." opposed to "I must do my homework." The difference is there, though one may regard it as vague.

<He (to himself): "I must buy that new single by MissP."

Later:

She: "Are you going to buy MissP's new single?"
He: "Oh yes, thank you for reminding me. I knew there was something I had to do.">

Still past. The plan was made before the moment of speaking.

I've been told  that must/have to are used interchangeably. However, my English text books say that the difference is some context considerable. What is your view on this?

I don't know of any context in which the difference might be described as considerable.  Perhaps you should post some examples.

My view on modals in American English is shown in Post:67841.

The basic uses are as follows:

1.  must is used to indicate an inevitable logical conclusion.

The lights are on.  He must be at home.
I can't find my pen.  I must have lost it on the way here.

2.  have to is used to show an obligation.

I have to take a final exam next Tuesday.
Mary had to go to the bank, but she'll be back in a few minutes.

If you use only the two described above in ordinary conversation, you will match American usage nearly 100% of the time.
______

3.  That said, on occasion, but not often, have to replaces must in the meaning described above (1.).  It gives the impression of being more casual in tone, while in some cases simultaneously insisting somewhat more strongly on correctness of the conclusion.  (Using must in the examples below is not wrong.)

Her ex-husband was going to the same party.  That has to be why she turned down our invitation.
This has to be the answer.  All the other choices are wrong.

4.  Also, on occasion, but more frequently than the substitution just described (3.), must substitutes for have to described above (2.).  It gives the impression of belonging to more official language - and/or of being more formal in tone.  Consequently, it is used more often in rules and instructions than in ordinary conversation.  (Using have to in the examples below is not wrong, but sounds less official.)

All employees must wash their hands before returning to the kitchen.
The application must be filled out and signed before September 1.

(As for analyzing what thought you wish to express in terms of subjectivity and objectivity, note that there is little agreement on what to call subjective and what to call objective.  Consequently, in my opinion, the net result of such analysis while in the act of trying to speak in a real conversation will almost certainly leave you tongue-tied, no matter how interesting the topic may be as arm-chair philosophy.)

CJ