What you'll learn
By the end of this lesson, you'll be able to express obligation, permission and prohibition clearly with modal verbs.
When do I use it?
- Obligation — have to / must: "I have to finish this report." "You must wear a seatbelt."
- No obligation — don't have to: "You don't have to come if you're busy." (it's optional)
- Permission — can / may: "Can I leave early?" "You may sit down." "You are allowed to park here."
- Prohibition — mustn't / can't / not allowed to: "You mustn't smoke here." "You can't park there."
The key contrast for Spanish speakers: mustn't = prohibition (not allowed), while don't have to = no obligation (optional). They are not the same!
How do I form it?
have to + verb → obligation don't have to + verb → no obligation can / may + verb → permission mustn't / can't + verb → prohibition
- We have to wear a uniform at work.
- You don't have to book a table.
- Can I open the window?
- Students mustn't use phones in class.
Common mistakes for Spanish speakers
"Tener que" and "deber" both exist in Spanish, so "mustn't" gets confused with "don't have to": "no tienes que venir" means "you don't have to come" (optional), but "you mustn't come" means "no puedes venir" (forbidden). Spanish speakers also say "I must to go" (wrong — no "to" after must) and use "can" for permission to such an extent that the past and future need "be allowed to": "I was allowed to go", "you will be allowed to go", not "I could go" for one specific permission.
Mustn't = forbidden. Don't have to = optional. Can = permission. Have to = obligation.
Practice
Multiple Choice
B1Which sentence means 'it's optional'?
Fill in the Blank
B1You smoke here — it is forbidden.
Hint: Prohibition → mustn't + verb.
Find the Error
B1Multiple Choice
B1Which sentence asks for permission?
Practice Speaking
Apply what you've learned in a real conversation with AI feedback.
Level: B1