These questions ask you to choose or build an affirmative obligation phrase. The reliable clues are the time frame, the subject and the grammar immediately after must or have to.
Choose the time frame before the form
First decide when the obligation applies. A finished event with a marker such as yesterday, last week or during a completed disruption calls for had to followed by the base form of the main verb. A future requirement uses will have to followed by the base form. For an obligation in force now, affirmative must and have to are both standard unless the sentence gives some other grammatical reason to choose between them. The common tendency to associate must with the speaker and have to with an external rule can add nuance, but it is not a dependable right-or-wrong contrast by itself.
| Signal | Form to build |
|---|---|
| A present rule or requirement | Use must plus the base verb, or the present form of have to plus the base verb. |
| A completed past obligation | Use had to plus the base verb. |
| A future obligation | Use will have to plus the base verb. |
| A third-person singular subject in the present | Use has to rather than have to; the main verb after to remains in its base form. |
Then check the shape of the phrase. Must is a core modal, so it is followed directly by the base verb: there is no to, no -ing form and no third-person -s on that verb. Have to behaves differently because have carries tense and subject agreement. Once have, has, had or will have is settled, keep to and use the base form of the main verb.
The traps
| Tempting form | How to reject it |
|---|---|
| Must followed by to, an -ing form or an -s form | These endings import grammar that a core modal does not allow. Put the base verb directly after must. |
| Present must or have to beside a finished past marker | The meaning may still sound like necessity, but the time form is wrong. A completed past obligation needs had to. |
| Had to beside a requirement explicitly in force now | Had to moves the obligation into the past. Recheck the stated time before choosing a familiar obligation phrase. |
| Have followed by a past participle without to | That builds a perfect construction describing an action, not the have to pattern for obligation. Look for to before the required base verb. |
Try it
Complete the sentence with the past form of have to and the exact verb in brackets: Because the wind changed suddenly last night, the pilot ___ the balloon flight before dawn. (cancel)
Select all that apply: which sentences correctly express the same current obligation at the ceramics studio?
- Potters had to wear eye protection while using the wheel.
- Potters must wear eye protection while using the wheel.
- Potters must to wear eye protection while using the wheel.
- Potters have to wear eye protection while using the wheel.
The annual licence rule applies to one operator: Each market trader ___ before selling prepared food.
- have to renew the hygiene certificate
- has to renew the hygiene certificate
- has renew the hygiene certificate
- must to renew the hygiene certificate