Must not and am/is/are not to are prohibitions (negative commands).
They are Present and Future in meaning; but future obligation can be made more precise with the form shall have to.
Past Tense :
Had (got) to is an obligation in the past.
Was/were not to is a prohibition in the past: it occurs mostly in reported speech.
To express the absence of obligation or necessity to do something (that is, the opposite of must), the form need not is used.
You must go now.
No, you needn't go just yet, you can stay a little longer.
Alternative forms are haven't go to, don't have to, don't need to.
Future is the same as the above, but we use shan't (won't) have to and shan't (won't) need to if we wish to be more precise.
Past forms are hadn't got to, didn't have to, and didn't need to.
There is usually a difference of meaning between the present tense forms must and have to in affirmative statements :
Must expresses obligation or compulsion from the speaker's viewpoint.
Have to expresses external obligation.
Compare the following pairs of situations, where these two forms are used in their natural context.
- You must go now. (I want to go to bed.)
- What a pity you have to go now. (It's time for you to catch your train.)
- We must begin before five (or we shan't finish in time for our supper).
- We have to begin before five. (That's the time arranged.)
- They must take it away. (I won't have it here any longer.)
- They have to take it away. (They've been told to do so.)
- He must stay the night. (I/We press him to do so.)
- He has to stay the night. ( He can't get back tonight.)
- He must move the furniture himself (for all I care; I shan't help him.)
- He has to move the furniture himself (poor chap; he's got no one to help him.)
- You must call me 'Sir'. (I like it that way.)
- You have to call me 'Sir'. (That's the regulation address.)
- You must change your shoes. (I won't have you in here with muddy feet.)
- You have to change your shoes. (... such is the custom on entering a mosque.)
Am/is/etc. to is used for definite commands or prohibitions. This form, or must, is used for instructions on notices or orders (have to is never used here).
- Passengers must cross the lines by the footbridge. (The railway company instructs them to.)
- Porters often have to walk across the lines. (The nature of their work compels them to.)
- All junior officers are to report to the colonel at once. (Military order)
- Soldiers have to salute their officers. (Such is the custom.)
In questions and negatives the present tense forms :
| Do I have to...? | Do I need to...? |
| I don't have to... | I don't need to... |
This is a preference, not a rule, and except where one of the above aspects requires emphasis, all four forms are interchangeable. For some speakers there is a difference of meaning between Need you goo shopping this afternoon? (which may sound like a protest) and Do you need to go shopping this afternoon? (a simple enquiry).
The forms of have to and need (with and without the auxiliary do) can be compared in the following examples, where the idea of one special occasion or of habit is strongly present.
- Must you get up early tomorrow morning? (Have you go to...)
- Do you have to get up early (every morning)?
- Must I show it to him now? (Have I got to...)
- Do I have to show my pass every time I go in?, or
- Do I need to show my pass every time...? (I hope it isn't necessary.)
- You needn't do it just now.
- You don't have to do it every time you see him. (You don't need to.)
Need is used wherever there is a strong element of negation or doubt, or when the speaker seeks or expects a negative answer.
Examples :
- Must she come tomorrow? (open question)
- Need she come tomorrow? (hoping for negative answer)
- Must I be present? (Do you want me?)
- I wonder if I need be present. (statement of doubt)
didn't need to : It wasn't necessary, so probably not done.
needn't have : It wasn't necessary, but done nevertheless.
Both are opposites of I had to, with the above difference of meaning.
My tea was already sweetened, so I needn't have put any sugar in.
But I did, and made it too sweet.
My tea was already sweetened, so I didn't need to put any sugar in.
I drank it as it was.
I didn't need to change my suit (=didn't have to).
So I went in the clothes I had on.
I needn't have changed my suit.
But I did! I see now that it wasn't necessary.