Walter Montego: I don't really get it. The link Marfitalu gave says that when there's external force, you have to use have to but when it's an option and it is you who decides whether or not to do the job, you may use must. Yet you say that if your wife gets upset, which I believe is an external force making you do the job, you use must.

I went back to those books I were studying, and surprisingly at the end of one of them found some grammar notes I had missed till now. There are a few points about Have to and Must, and Should and Ought to. See if you agree with them:

Should

1) Advice: You should go and see Casablanca. It's a brilliant old fim.

2) Obligations: I should get my father a card. It's his birthday tomorrow.

3) Probability: If the train's on time, we should arrive at 3:30.

Ought to

1) Advice: You ought to stop smoking. It's really bad for you. (Ought to expresses less personal advice than should.)

2) Obligations: I really ought to pay the telephone bill tomorrow. (Ought to here indicates that the speaker probably won't pay the bill tomorrow)

3) Theory: John ought to be here by now. (Ought to here means was due to, or it is expected)

Must

Must + infinitive is used for strong obligations which express the authority of the speaker or writer. So it is used:

1) For formal rules or laws: Passengers must fasten their seat belts for take-off.

2) For suggestions, advice or recommendations that the speaker feels strongly about: You must come to my party. Everyone's gonna be there.

have to

Have to + infinitive is used for strong obligations which express the authority of a third person, rather than the speaker or writer. So it is used:

1) When the speaker wants to show they ar enot responsible for imposing the obligation, or do not agree with it: I'll be late home tonight. I hav eto work late. My boss says so.

2) When the speaker or writer is reminding someone about a rule or law: I'm sorry, but you hav eto wear a seat belt in the back of cars now.

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So according to what I typed above, if your wife is not going to be mad at you, you can say:

I should buy a loaf of bread on my way back.

If she gets mad at you, you can say:

I have to buy a loaf of bread on my way home.

And if you yourself will get upset if you cannot satisfy what your wife has asked you to do, you can say:

I must buy a loaf of bread on my way home.

How mush do you think what I said is right according to what you think you really would say in these situations?

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Yes, I know that people don't care for their choice of vocabulary so much, but we have a spoken English and a proper English, which you may call book English. I think I, or any non-native speaker is supposed to first learn to speak the language quite properly, then try to speak it as the speakers do.

When I speak farsi, I sometimes do notice that the sentence I'm saying is wrong regarding Farsi structure of sentences, but I just know that the one I'm talking to understands me without laughing at me or even asking why I've made the mistake.

The word 'whom' has cost me many marks in my exams, so now I'm glad I know its difference with who. :-)