Expressing obligation and necessity with must / have to / don't have to / mustn't
Must + base verb; Have to + base verb
Must not (mustn't) + base verb; Do/does not have to + base verb
Must + subject + base verb? / Do you have to + base verb?
💡 Mustn't = prohibition. Don't have to = not necessary. Past: had to. Use must for deductions (strong belief).
✓You must finish this today.
✗You must to finish this today.
No 'to' after must.
✓We have to wear badges.
✗We must to wear badges (external rule).
Have to fits external obligation; no 'to' after must.
✓You don't have to call.
✗You mustn't call if you don't need.
Don't have to = not necessary; mustn't = prohibition.
✓She must be at home.
✗She has to be at home (deduction).
Use must for strong logical deduction.