Casa ESL · grammar guides · level B1
Must, Have To, and Should — Obligation and Advice
Three verbs run the world of rules: must (strong, often personal), have to (strong, often external), and should (advice). Their negatives are where meanings flip dramatically.
Obligation: must vs have to
Both are strong. Must often comes from the speaker: "I must call my mother" (I feel it). Have to often comes from outside: "I have to wear a uniform" (the rule says). In everyday speech, have to does most of the work; must sounds formal or written ("Passengers must keep seatbelts fastened").
Past obligation is always had to — must has no past: "We had to wait two hours."
The negatives don't match
Mustn't = prohibited: "You mustn't smoke here" (it is forbidden). Don't have to = not necessary: "You don't have to come" (optional!). Mixing these changes rules into options and options into rules — the classic B1 disaster.
Should: advice and expectation
"You should see a doctor" (advice). "The bus should arrive soon" (expectation). Stronger advice: ought to (same meaning), had better (with a hint of warning): "You'd better hurry."
| Form | Force | Example |
|---|---|---|
| must / have to | obligation | You must show ID. / I have to work late. |
| mustn't | prohibition | You mustn't park here. |
| don't have to | no obligation | You don't have to pay — it's free. |
| should / ought to | advice | You should rest. |
| had better | urgent advice | We'd better leave now. |
Common mistakes
✓ You don't have to pay — it's free.
Mustn't forbids; don't have to means optional.
✓ I must go.
No to after must.
✓ She had to work yesterday.
Must has no past form.
Practice
- Students ___ (not/use) phones during the exam. (forbidden)
show answer
mustn't use - You ___ (not/bring) anything — we have everything. (optional)
show answer
don't have to bring - I ___ (work) last Saturday. (past obligation)
show answer
had to work
FAQ
What is the difference between must and have to?
Both express strong obligation. Must often reflects the speaker's own feeling or formal rules; have to usually reports external requirements. In the past, both become had to.
What is the difference between mustn't and don't have to?
Mustn't means something is forbidden. Don't have to means it isn't necessary but is allowed. They are opposites in force, not synonyms.
When should I use should?
For advice and recommendations (you should sleep more) and for expectations (the parcel should arrive today).
Want this to actually stick?
One-on-one lessons with a real teacher, or free worksheets for this level — your pace, your goals.
More guides: Present Simple · Past Simple · Present Continuous · Present Perfect