Expressing obligation: must, have to, need to, should, ought to
Subject + modal + base verb
Must not (prohibition) vs Don't have to (no obligation)
Must I...? Do I have to...? Should I...?
💡 Mustn't = prohibited. Don't have to = not necessary (big difference!).
✓You mustn't smoke here. (prohibited)
✗You don't have to smoke here. (wrong meaning)
Mustn't = not allowed; don't have to = not necessary.
✓I have to go now. (external)
✗I must to go now.
No 'to' after must; have to needs 'to'.
✓You should see a doctor.
✗You should to see a doctor.
No 'to' after should.