Use must, have to and need to for necessity; mustn’t for prohibition; and don’t have to for something optional. Use should or ought to when you mean advice or what is desirable. The biggest practical mistake is confusing “You mustn’t come” with “You don’t have to come”: the first says not to attend; the second gives you a choice.
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Choose the meaning before the verb
| Meaning | Useful form | Example and interpretation |
|---|---|---|
| Necessary or required | must / have to / need to | We have to submit the form today. Today is the deadline. |
| Not allowed | must not / mustn’t | You mustn’t share this password. Sharing it is prohibited. |
| Not necessary | don’t have to / don’t need to | You don’t have to print the ticket. A digital copy is accepted. |
| A good idea or the right thing to do | should / ought to | You should save a copy. This is advice, not a requirement. |
| A bad idea | shouldn’t | You shouldn’t leave your preparation until the last minute. |
Strictly speaking, must and should are modal auxiliaries. Have to and ordinary need to express similar meanings but use different grammar. Learning their sentence patterns together helps you avoid forms such as “She musts” and “Does she must?”
Must and have to: a useful distinction, not a fixed rule
Must often presents a requirement from the speaker’s point of view: “I must call my sister; I promised I would.” Have to often reports a requirement or circumstance: “I have to leave at six because the last bus goes then.”
However, must also appears in official instructions: “Visitors must show their booking confirmation.” Both forms can refer to outside rules. The choice depends on how you present the obligation, so do not treat “personal versus external” as an absolute test. Have to is a useful everyday way to ask about requirements: “Do we have to book in advance?”
The sentence patterns
| Form | Statement | Question |
|---|---|---|
| must + base verb | She must leave. | Must she leave? |
| have / has to + base verb | She has to leave. | Does she have to leave? |
| need / needs to + base verb | She needs to leave. | Does she need to leave? |
| should + base verb | She should leave. | Should she leave? |
| ought to + base verb | She ought to leave. | Ought she to leave? |
After must and should, do not add to or an -s ending. With have to, use has for he, she or it in statements, but return to have after does. Questions with ought to are possible, though should is generally the more natural conversational choice.
Mustn’t and don’t have to are different
Imagine an optional workshop with a rule against recording. The organiser says: “You don’t have to attend, but if you join, you mustn’t record the session.” Attendance is your choice. Recording is forbidden.
The distinction also changes a short answer. “Must I bring a laptop?” can be answered “No, you don’t have to; we provide one.” “No, you mustn’t” would mean bringing one is not allowed. Needn’t also means that something is unnecessary: “You needn’t bring a laptop.”
Obligation in the past and future
For a completed past situation, use had to: “Yesterday I had to work late.” For a specific future requirement, use will have to: “Next month we’ll have to use the new booking system.” Must can also refer to the future: “I must send this tomorrow.” It simply does not have an ordinary past form for past obligation.
| Time or meaning | Example | What it tells us |
|---|---|---|
| Past requirement | I had to pay a deposit. | The deposit was required. |
| No past requirement | I didn’t have to pay a deposit. | There was no requirement; this form alone does not tell us whether I paid. |
| An unnecessary completed action | I needn’t have paid a deposit. | I paid, but paying was unnecessary. |
| Future requirement | I’ll have to pay a deposit. | I expect the requirement in the future. |
Do not use must have paid to mean had to pay. “She must have paid already” normally expresses a deduction: you conclude that payment happened. For the tense contrast, review base forms, past forms and participles.
Should and ought to: advice and expectations
“You should apologise” and “You ought to apologise” can both express what the speaker considers the right action. Ought to is not restricted to moral duties: “The parcel ought to arrive tomorrow” expresses an expectation. Neither word automatically turns advice into a compulsory rule.
For a regret or criticism about a past action, use should have + past participle: “I should have checked the address.” In this example, the speaker regrets not checking it. Compare that with “I had to check the address,” which describes a requirement.
Practice: choose the form that matches the situation
Choose one of the two options in each item. Read the situation, not just the gap.
- Recording is prohibited. You ___ record this meeting. (mustn’t / don’t have to)
- Attendance is optional. You ___ attend. (mustn’t / don’t have to)
- There was a compulsory safety briefing yesterday. We ___ attend it. (had to / must)
- ___ she have to wear a uniform? (Does / Must)
- This is my advice, not a requirement: you ___ take a spare charger. (should / must)
- I bought a ticket, then discovered entry was free. I ___ bought a ticket. (needn’t have / mustn’t have)
- Use the future form: next year, students ___ apply online. (will have to / had to)
- “She must have left already” expresses ___. (a deduction about the past / a past obligation)
Answers and reasons
- Mustn’t: the instruction forbids recording.
- Don’t have to: optional attendance means there is no obligation.
- Had to: the compulsory event was in the past.
- Does: questions with have to use do or does in the present simple.
- Should: the speaker explicitly offers advice.
- Needn’t have: the purchase happened but was unnecessary.
- Will have to: the instruction asks for the future form.
- A deduction about the past: must have + participle expresses a conclusion here.
Try it yourself: write three sentences about a course or workplace: one requirement, one prohibition and one optional activity. Then check whether each sentence gives the reader the intended choice.
Grammar references: Cambridge English Grammar Today on must and have to and ought to.
