Modal Verbs


I can swim but I can't fly. (I'm able to swim but I'm not able to fly.)


Must.- 100% sure, no doubt about it.

Might.- 50% sure. You are hesitating.

Can't.- 0%. No way. That's impossible



1.- Have to / Must

Similar but a bit Different

  • I have to finish my homework. If not my mother won't let me go out. (Obligation)
  • I must finish my homework. I don't want to stay at home all they long. (Feeling)
  • People under 18 must wait in the other room. (Sign)


If you hesitate or doubt, use have to. It is always correct!


2.- Don't have to / Mustn't


Be careful, here meanings are totally different:


  • I don't have to finish my homework today. I still have time till the end of the month. (There is no need)
  • I musn't finish my homework today. The teacher didn't give us permission.
  • You musn't drive a car if you are under 18. You are not allowed by law.
  • Passengers must not smoke in the cabin. It is forbidden by law.


3.- Must / Should


  • You must finish your homework today! It's a command.
  • If you go to Paris, you must visit the Louvre. You cannot miss it!
  • You should finish your homework today, if so you can go out! It's a recommendation. You are giving advice or opinion.

4.- Shouldn't


  •  You shouldn't eat so many sweets.- That's my opinion but ... I wouldn't do that. I think that's not ok. That's not correct.




1.- Personal Opinion.- Are you tired?  Maybe you should go home and take a rest.

2.- Objective Opinion.- Ann, look at your face. You ought to go home and take a rest.




1.- May I go in?


2.- May I erase the blackboard?