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I was asked by a student to explain the difference between must, ought to, need to, have to and should. Could someone give me a clear, concise explanation? Thanks in advance!

This is a big area! Some of the words in your list are modal verbs and others not. If you haven't already bought "Practical English Usage" by Swan I strongly recommend you get a copy and study it.

It seems the connection between the words you've listed is obligation or necessity.

Should and Ought to don't seem to me to belong in this group because they are used to give advice.

Must expresses internal obligation (the speaker is expressing a personal opinion) whereas have to expresses external obligation (it's not a personal opinion).

Need to expresses necessity, but not obligation.

Here's a link which might help. http://ccc.commnet.edu/grammar/modals/modals_frames.htm

Iain

dduck wrote:This is a big area! Some of the words in your list are modal verbs and others not. If you haven't already bought "Practical English Usage" by Swan I strongly recommend you get a copy and study it.

It seems the connection between the words you've listed is obligation or necessity.

Should and Ought to don't seem to me to belong in this group because they are used to give advice.

Must expresses internal obligation (the speaker is expressing a personal opinion) whereas have to expresses external obligation (it's not a personal opinion).

Need to expresses necessity, but not obligation.

Here's a link which might help. http://ccc.commnet.edu/grammar/modals/modals_frames.htm

Iain

Well, I agree this is a big area, but I sure don't understand your explanation of must and have to. Of course, I could be convinced, but I'd like to see some sentences showing this.

For example, what about "The store is closing. You must leave now." Is that an "internal" and not "external" obligation?

I have never seen this distinction made in any of the grammar books I've looked at. A small corpus would be helpful.

Lorikeet wrote:I have never seen this distinction made in any of the grammar books I've looked at. A small corpus would be helpful.

Father: You must do what your mother tells you.
(The father forcefully tells his son what he believes, using "have to" would sound strange here. You could use "have to" if there's less emotion involved and if the father's just playing along but not agreeing with his wife.)

Father: You have to take your medicine.
(The father is saying that the doctor wants the son to take the medicine. If you used "must" the father would be expressing his firm belief that the doctor's advice is correct).

You must be careful (I'm telling you)
He says you have to leave if you don't shut up (don't blame me, it's not my idea!)

I read this recently in "The English Verb", but Michael Lewis, but it's not the first time I seen it.

Iain

Well, I'm not yet convinced. To me, the "must" seems more formal. I don't use "must" very often in my normal speech. I can use "have to" with or without an emphasis. In your examples I can use "have to" and "must" pretty much interchangeably.
Father: You must do what your mother tells you. (Also, Father: You have to do what your mother tells you. You have no other option.)

Father: You have to take your medicine. (Also, You must take your medicine)

It is, of course, possible that there are some differences in usage between American English (mine) and other Englishes. I'll wait to see what everyone else thinks

Hi Lorikeet,

I have found that describing words or usages as "formal" or "informal" is (to me, at least) not very helpful for making clear distinctions. It always leaves me feeling a bit unsatisfied because I'm never sure what the dividing line is between what is formal and what isn't. (Of course, if I were being introduced to the Queen, I could figure out that my language ought to be formal, but there are so many other more ordinary situations that are rather more difficult to sort out with regard to formality.) So suggesting that must is more formal than have to seems (to me, at least) like it likely won't be of much help to your students.

Actually, Iain gave an excellent, if concise, survey of the situation in his first post above. (The only part of it I didn't like was his link to a website which is full of nonsense.) I understand your skepticism concerning the distinction between must and have to, but what he said is, I think, right on the money. But let's see if I can expand on it just enough to convince you of his accuracy.

What he means by "internal obligation" and "external obligation" is sometimes hard to imagine. I assume that it is uncontroversial that both terms (and indeed, the whole list Surrealia originally submitted) are concerned with the concept of 'obligation', or perhaps we could say, 'necessity'. The question is, "What (or who) is the source of the obligation?" (Obligation has to come from somewhere.) Is what I'm talking about necessary because I say so, or is it because of someone or something else other than me?, me being the speaker or writer. I don't think it has anything to do with forcefulness, just as you have pointed out. Both must and have to can be forceful or not so much so, depending on intonation and/or manner of speaking. But consider your example:

Father: "You must do what your mother says."

or

Father: "You have to do what your mother says."

I believe the distinction between the two is that in the first sentence, the Father is asserting that it is necessary that (the child) do what her mother says because I say so (too). In the second example, however, Father is suggesting that it is necessary because of some outside reason (such as, in this particular case, simply the fact that your mother said so).

Try another pair:

You must get a haircut! (I say so!)
You have to get a haircut. (School rules don't allow long hair.)

Try a series of example pairs of your own, and try to extend this distinction to them. See if it doesn't work for you.

Larry Latham

Regarding the website I chose - I looked round a few locations, but there's so much vagueness written about this subject that I found it hard to post anything I liked. If anyone can provide a good link, I'll be sure to make a note of it!

Iain

Ah, native speaker intuition is so much fun! I can't see any difference between "must" and "have to" that would lend itself to any kind of explanation I could give my students. I usually say that "have to" is more common in speech in this part of the country.

How about "I have to go" (This is internal--I gave myself a time limit and I'm sticking to it.)

"I have to go" (The place is closing and they told me to get off the phone and leave.)

"I have to go" (The place is closing and they told me to get off the phone and leave.)

Precisely, Lorikeet! Note that the reason you have to go is that the place is closing. It is not your internal decision, but one driven by outside influences.

If you were to decide all by yourself that it is necessary that you go, you most likely would say: "I must go."

Larry Latham

No no, you missed my point! (my bad )
The first example should have been beefier I guess.

I have to go now. (I decided to allow myself 2 hours to be at this party, and my two hours is up. It is completely internal.) The second example (the one you chose to quote) was external. I was trying to show that I can use "have to" for both of them, but I wasn't explicit enough in my first example.

I would never say "I must go." Alas, it sounds stilted to me.

OK Lorikeet. I would never dare tell you what you should say.

I'd be willing to bet some money, though, that most native speakers, with or without really knowing it, would use must or have to in ways that I could explain using Iain's logic. Actually, as he mentioned too, I read about it first in Michael Lewis' The English Verb. A wonderful book. I heartily recommend it to all who would teach the English verb system.

Larry Latham

"have to" and "must" are often interchangeable.

Sometimes they're not.

Look at "I have to see you sometime". This is almost certainly work related; there is definitely some particular thing that you need to discuss.

"I must see you sometime". Probably suggests that you are a good friend and want to meet the person for the pleasure of it.

"I have to see the boss tomorrow" means he has probably called you in, or there is some important matter.

"I must see the boss tomorrow" suggests that you have a personal stake in whatever you wish to discuss.

Formality has nothing whatsoever to do with it.

Incidentally you would normally say "I have to go now" because you want to give your host the impression that you would dearly like to stay at her party until dawn the next day and it is only this unavoidable prior obligation that is forcing you to leave. "I must go now" might give the host the idea that you actually are rather glad to leave his party

Stephen Jones,

I totally agree with everything you have said here, except the first thing you said.

"have to" and "must" are often interchangeable.

Sometimes they're not.

...because everything else you said is evidence that what you said first is not true. Every "have to" you mentioned implies that there is an outside influence necessitating the action. Similarly, every "must" you used implies that the source of the necessity is the speaker himself. Thus they are not interchangable--they have different meanings.

Larry Latham

...besides, dear readers, let's not forget that the typical behavior of these two terms in negative situations is quite different--again suggesting that they are not the same.

You can say, "...don't have to"

But you cannot say, *"don't must"

You can say, "...must not"

But it would be unusual to say, "...have to not"

The reasoning for all this can be neatly explained if you keep in mind that the source of necessity for "have to" is external to the speaker, and the source for "must" is the speaker himself.

And, don't I remember Iain mentioning that one of these is a modal auxiliary and the other is not? Another reason to consider them not interchangable.

Larry Latham

Somehow the difference between "have to" and "must" must have gotten lost on the way to California! My kids can't see any difference either. (I know, it's probably my fault.)