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Dear teachers,

- What is the difference between HAVE TO and MUST?

- Could HAVE TO express a greater certainty from the speaker than MUST?

1) a) There HAS (got) TO BE a mistake.

b) There MUST BE a mistake.

2a) My friend MUST BE ill = Present; strong deduction: I am sure he is ill?
b) My friend HAS TO BE ill = Present; It’s more certain than with must ?

c) My friend HAD TO BE ill = Past; expresses a certainty?
d) My friend MUST HAVE BEEN ill. = Past: expresses a certain deduction ?

e) My friend OUGHT TO BE ill. (correct ? ) = Present : strong assumption ?
f) My friend SHOULD BE ill. (incorrect ?) = Present : weaker expectation / assumption
than “ought to” ?

- What’s the difference between deduction / expectation / assumption ?

Thank you in advance for your help.
Hela
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Hello,
"have to" usually expresses an obligation from the outside, eg rules, law etc... whereas "must" expresses a "personnal" obligation. Moreover, "must" also expresses a logical obligation: "It's snowing, it must be cold outside."
1)a) is stronger than 1)b), which means that, all considered, something is wrong.
2)a) means that, say because she's not at work today, then the logical explanation is that she's ill
2)b) is nearly impossible, because it would mean that she is obliged to be ill.
Most probably a teacher will explain it more clearly than I've just done!
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Final observation: The recommendations above are based on American English, but I would not be surprised if they were valid for all other varieties of English as well. In everyday conversation in American English, the "M"-modals (may, might, must) are almost exclusively the ones used for logical deductions and similar matters. "have to", "should", and "can" are more used for strong obligation, weak obligation, and permission or ability, respectively.

Interesting comment! Thank you, CJ

paco
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Based on what I hear daily, I would like to make a recommendation or two. If you start with these first and master them, expanding out into the additional meanings will be less of a problem than if you try to learn all possible meanings at once, which can get somewhat confusing.

1a. Use "must" only for logical deductions. Paraphrase: "Given the evidence, the only logical conclusion is that ..."
1b. Use "must have" for logical deductions about the past.

She's unable to get up out of bed. She's sneezing and coughing. She looks miserable. She must be very ill.
She was unable to get up out of bed. She was sneezing and coughing. She looked miserable. She must have been very ill.

There is no need to use "have to" for this meaning. Know it passively when you hear it or read it.

... She has to be very ill.
... She had to have been very ill. (These sound "klunky" to me. -- inelegant )

2. Use "have to" only for strong obligations. Paraphrase: "It is a duty (for someone) to ..."

Janice has to get up at five o'clock tomorrow. (obligation in the present and onward)
Janice had to get up at five o'clock yesterday. (obligation in the past and onward)

There is no need to use "must" for this meaning. Know it passively when you hear it or read it.

Janice must get up at five tomorrow.
*Janice must get up at five yesterday. (The past is unavailable with the meaning of "duty" or "obligation".)

3a. Use "should" for weak obligations, in other words, to advise or strongly suggest". Paraphrase: "It is advisable (for someone to) ..."

You should study English every day. (It is advisable for you to study English every day.)

There is no need to use "ought to". It has the same meaning and can be added to your vocabulary later, if you wish. It's not used as much as "should", anyway. Know it passively when you hear it or read it.

3b. "should have" differs considerably from "should" because it places the advisability in the past, and implies that the action advised was not carried out. It says that someone had an obligation or duty to do something, but did not.

You should have warned me that the water was boiling hot before letting me put my hand in it. (Your duty to warn me was in the past. You did not do your duty. My hand got scalded! Ouch!)

"ought to have" is the lesser used equivalent of "should have".

4. Use "will probably" for expectations. "will probably" is a good way to say that something is expected (as a logical deduction from the given evidence). This is a weaker form of "must".

He has been driving for five hours, so he'll probably be in Los Angeles by now. (He is expected to be in LA / I expect him to be in LA)

There is no need to use "should" in this context, though it has the same meaning. Know it passively when you hear it or read it.

He has been driving for five hours, so he should be in Los Angeles by now. (It is to be expected that he is in LA by now.)

Final observation: The recommendations above are based on American English, but I would not be surprised if they were valid for all other varieties of English as well. In everyday conversation in American English, the "M"-modals (may, might, must) are almost exclusively the ones used for logical deductions and similar matters. "have to", "should", and "can" are more used for strong obligation, weak obligation, and permission or ability, respectively.

I think maybe that's enough for one post!

Emotion: smile
 paco2004's reply was promoted to an answer.
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What's a wonderful explanation! There is nothing to add. Thank you, Jim. Emotion: smile