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There are two typical situations: when someone asks for a permission or for a obligation. I am totally confused, because there seems to be two ways to ask and to reply, and I don't know which one is right.

PERMISSION:

Q1. Can I go play?

Q2. May I go play?

N1. No, you must not.

N2. No, you may not.

P1. Yes you can.

P2. Yes you may.

OBLIGATION:

Q1. Do I have to go to school?

Q2. Must I go to school?

N1. No, you don't have to.

N2. (I guess there is no second possibility)

P1. Yes you have to.

P2. Yes you must.

Which one of the two forms is correct?

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Just want to add that I've googled the Internet for quite a while, and there were no clear answers, especially for NEGATIVE forms. I've added questions and positive forms, because it might happen that questions, negative and positive forms are somewhat related, e.g. Q1-N1 is OK, but Q1-N2 is not. –  Pygmalion Nov 24 '12 at 20:45
    
What exactly is your question? Except where the meaning is different (e.g.: "Must I go to school? No, you don't have to."), people will usually reply with the verb you used to ask. –  Peter Shor Nov 24 '12 at 21:10
    
...or they will interpret the question based on context, so Q1 might be assumed to be asking about permission rather than simple ability, and get answer N2. I don't really know what the question is here either. –  Andrew Leach Nov 24 '12 at 21:11
    
For a start, I would like to know if all these forms are actually correct. I understand that in PERMISSION section "Q1-P1", "Q2-P2" are more likely than "Q1-P2" and "Q2-P1", what about "Q1-N1" or "Q1-N2" - which is more likely or more correct? –  Pygmalion Nov 24 '12 at 21:17
    
@WillHunting That too. I'd say I am relatively good for a non-native English speaker, but I got completely confused when explaining this stuff to my daughter! There are so many forms that even I got confused. –  Pygmalion Nov 24 '12 at 21:20
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closed as not a real question by Carlo_R., tchrist, Andrew Leach, MετάEd, Mitch Nov 25 '12 at 22:39

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

3 Answers

up vote 2 down vote accepted

Can I do this? Yes, you can. No, you cannot.

May I do this? Yes, you may. No, you may not.

Both are correct when asking for permission, but using may sounds more polite than can.

Do I have to do this? Yes, you have to. No, you do not have to.

Must I do this? Yes, you must. No, you need not.

Both are correct when asking about necessity, but using must sounds stronger than have to.

Note that must not means one is forbidden from doing something, not that one is not required to do something.

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The most problematic for me are negative forms. Can you include them in your answer? –  Pygmalion Nov 24 '12 at 21:28
    
Now I am even more confused. I thought that "you can not" means only "you are not able to". "you can not" also means "you are not permitted to"? –  Pygmalion Nov 24 '12 at 21:34
    
So suppose you say (without question) "I can't go" it means both you are not possible to do that (because I have broken leg), and not that you are banned to (parents don't let me)? –  Pygmalion Nov 24 '12 at 21:43
    
It is now clearer to me, but one more (and last) question please. So my daughter is learning in school obligation and prohibition forms. They use "you must wash your hands" and "you must not pick your nose". The prohibition "must not" form sounds very strange to me, and since negative answer when asking for permission is actually prohibition, where "must not" fits into this scheme of questions and answers? –  Pygmalion Nov 24 '12 at 21:51
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So I conclude "must not" is (to a certain degree) equal to both "can not" and "may not", and does not have it's own question form. –  Pygmalion Nov 24 '12 at 21:57
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It's almost simple, but not quite, because must is devious.

  1. In a request for permission:

    • In formal contexts , may is preferred and can strongly deprecated.
    • In informal contexts, can is virtually universal now and may almost defunct.
  2. In an inquiry about obligation:

    • in formal contexts, must is preferred and have to strongly deprecated.
    • in informal contexts, have to is virtually universal now and must almost defunct.
  3. In responses, as Peter Shor tells you, your interlocutor will normally respond with the same form you employ. (Well, actually, in the specific cases you offer, Mom will usually just say "Yes" or "No"!), context insuring that the auxiliary bears the same significance as it did in the request/inquiry, EXCEPT THAT:

  4. Must not means something entirely different, viz. "You are prohibited from going to school." In this case you would probably respond (maintaining the formal register) "No, you need not."
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Another way to look at this is that may means "do I have permission to do something?" while can means "am I able to do something?"

This means there are many other possible negative answers to "Can I do this?" besides "No, you must not," such as:

Can we stop by the store before we go to the theatre?
No, we don't have enough time – the show starts in 10 minutes!

Can you buy me a new Lexus for Christmas?
No, I don't have enough money for that.

Can I move that boulder to the other side of the yard?
Not without some help; it's too heavy, and you're not strong enough.

The same is true for may, which is asking for permission, rather than inquiring about ability:

May I move that boulder to the other side of the yard?
No, I like it where it is. Leave it there.

In short, I think your question is based on a faulty premise: that there are "two ways to ask and to reply." In fact, there are a myriad of ways to ask and reply, and perhaps some of your confusion stems from the fact that the words and the language are more flexible than you presume.

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