These questions ask whether a negative statement expresses a prohibition or merely removes an obligation. Decide what freedom remains before you choose the grammar.
Ask whether the action is allowed
Begin with meaning, not with the negative word. If a rule, warning or authority says that the person is not allowed to perform the action, use must not or mustn't. If the person may perform the action but is free to leave it undone, use do not have to or don't have to. Optional, voluntary and not required are strong signals for the second meaning; forbidden, prohibited and explicit safety restrictions signal the first.
| Meaning check | Form to use |
|---|---|
| The action is not allowed | Use must not or mustn't plus the base verb. |
| The action is allowed but not required | Use do not have to or don't have to plus the base verb. |
| A third-person singular subject has no obligation | Use does not have to or doesn't have to plus the base verb. |
A two-question check keeps the contrast clear. First ask, ‘May the person do it?’ If the answer is no, the sentence needs the prohibition form. If the answer is yes, ask, ‘Is the person required to do it?’ A second no means there is no necessity, so use the appropriate form of do not have to. Finally, inspect the main verb: must not takes the base verb directly, while do not have to keeps to before the base verb.
The traps
| Tempting move | How to reject it |
|---|---|
| Using don't have to for an explicit ban | This only says the action is unnecessary and still leaves it available. A prohibition removes permission, not just necessity. |
| Using mustn't for an optional or voluntary action | This changes freedom into a ban. When either choice is permitted, remove the obligation with don't have to instead. |
| Putting to or an -ing form after must not | Must is a core modal and takes the base verb directly, even when it is negative. |
| Dropping to after don't have, or using haven't to | Standard British English builds absence of necessity with do plus not plus have to. Keep to before the base verb and let do carry subject agreement. |
Try it
Complete the prohibition with must not or its contraction and the exact verb in brackets: The powder is toxic and handling it is forbidden, so interns ___ it without sealed gloves. (touch)
Which sentence contains the meaning error in the optionality-versus-prohibition contrast?
- The solvent is highly flammable, so workers mustn't smoke near the open drum.
- Breakfast is included but optional, so guests don't have to order it.
- Friday's extra rehearsal is optional, so choir members mustn't attend it.
- The nesting area is closed, so walkers mustn't leave the marked path.
The wet-paint notice explicitly forbids touching the railing. The shop says keeping a paper receipt is optional because a digital copy is sufficient. Select all that apply: which sentences correctly restate those notices?
- You don't have to touch the wet railing.
- You mustn't touch the wet railing.
- Customers mustn't keep the paper copy.
- Customers don't have to keep the paper copy.