Modal Verbs of Obligation (must / have to / should)

What it means

Must, have to and should tell someone what is necessary or what is a good idea. Must and have to are strong: it is necessary. Should is softer: it is a good idea. Khmer speakers can think of ត្រូវតែ (must / have to) and គួរ (should); Chinese speakers can think of 必须 / 得 (must / have to) and 应该 (should).

How to make it

• must / should + base verb (no "to"): You must go. You should see a doctor.
• must and should never change: he must, she should (never "he musts").
• have to changes with the subject: I have to work, she has to work on Saturdays.
• have to changes with the tense: I had to wait (past), I will have to wait (future). Must has no past form, so use "had to" for the past.
• Questions and negatives of have to use do/does/did: Do you have to work? I don't have to go.

When to use it

• Rules, laws and signs (must): Passengers must show their tickets.
• Strong personal feeling, from the speaker (must): I must call my mum tonight.
• Obligation from outside, such as a job, law or timetable (have to): I have to get up at six for work.
• Advice and opinions (should): You should drink more water.

Compare

• must vs have to: must = the speaker or a rule says it is necessary; have to = the situation or someone else requires it. "I must study" (I decide) vs "I have to study" (the exam requires it).
• mustn't = it is not allowed: You mustn't smoke here.
• don't have to = it is not necessary (you can if you want): You don't have to leave the building.
• should = advice, not a rule: You should rest. (You can choose.)

Common mistakes

• ❌ You must to go. → ✅ You must go. (no "to" after must or should)
• ❌ She have to work. → ✅ She has to work. (have to changes with the subject)
• ❌ You mustn't come tomorrow. (when you mean it's optional) → ✅ You don't have to come tomorrow. (mustn't = forbidden)