modals: special difficulties (5)

5.- ABSENCE OF OBLIGATION/PROHIBITION

We all know that mustn’t is used to express prohibition, and needn’t or not have to for absence of obligation:

You mustn’t do it

You needn’t do it/you don’t have to do it

But, attention, if there was no need for you to do sth, but you did it nevertheless, needn’t have is used:

You needn’t have done it – no tenías que haberlo hecho  

and if there was sth that you had to do, but you didn’t do it, use should have and not must have, because must have indicates supposition:

You should have done it

Practice- 5

* Insert  needn’t or mustn’t according to whether the sentences express obligation or absence of obligation

a.- You ..................... cut the roses in the park; it’s forbidden.

b.- You ..................... tell him, I’ve already done it.

c.- You...................... wake me up; I’ve bought an alarm clock.

d.- You ..................... smoke in the office.

e.- Why did you order chicken as well? you ................... have ordered so much food

** Complete now with must have or should have according to whether you want to express supposition or sth that had to be done and wasn’t done

a.- “He didn’t phone me.” “He ........................ forgotten.

b.- “Sorry I dropped the glass”. “You ..................... been more careful.

c.- “A burglar entered my house last week”. “You .................... left the door open; you .......................... locked it.”

KEY TO PRACTICE -5

 * a.- mustn’t; b.- needn’t; c.- needn’t; d.- mustn’t; e.- needn’t.

** a.- must have; b.- should have; c.- must have; should have.