“Can” or “may” and “have to” or “must” [closed]

Question

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?

Answer 1

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.

Answer 2

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."

Answer 3

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.