Expressing Obligation/ No Obligation

OBLIGATION 
PRESENT  
Must and Have to: Generally, the difference between these depends on where the authority comes from:
You have to have a licence to own a gun ( an outside authority)   
You must come home at 10 o’clock ( the speaker’s authority)
I must study English (I’m telling myself)
I have to study English (It’s a duty)  
Should and Ought to: these suggest that the obligation is weaker, and may not be kept to:
I should talk to him (but I don’t have the courage) 
I ought to talk to him    
Is to/Are to: In formal and written language, they are used to express outside obligation, usually as a  part of a programme:   
Students are to arrrive by 9 o’clock 
Candidates are not to bring dictionaries into the examination. 
Had better: 
You’d better go home. 
She’d better not get up late again. 
 ***** MUSTN’T = prohibition.  You mustn’t smoke in the here.  
PAST 
had to (to express the Past tense of MUST and HAVE TO  
They had to pay at once  
Should have done/ Ought to have done: An event in the past did not take place, a mistake was made: 
You should have looked twice before turning left (you didn’t)  
He ought to have told her the truth (he didn’t)’
LACK OF OBLIGATION:  
PRESENT
Needn’t// don’t/doesn’t have to:
You needn’t bring your dictionaries
You don’t have to bring them
you don’t need to bring them
PAST
I didn’t have to go to school (it was unnecessary, I didn’t go)
I needn’t have gone to school (it was unnecessary, but I went) 
EXERCISE:
Rewrite the following sentences without changing the meaning of the original ones.Use the right modal.

1. It was silly of Barbara to be rude to her teacher.
2. It was a bad idea to ask him to do it. 
3. It wasn’t necessary to wait till midnight, so I went home.
4. It wasn’t necessary to wait till midnight, but I did it.
5. It’s a pity you didn’t write to her.
6. Come here!
7. Don't eat all the cake!
8. It isn't compulsory for her to work in the evening.
9. It was obligatory to write a poem.
10. It's compulsory to bring your books.(it's the rule)

Solutions:

1. Barbara shouldn't have been rude to her teacher.
2. You shouldn't have asked him to do it.
3. I didn't have to wait till midnight.
4. I needn't have waited till midnight.
5. You should have written to her.
6. You must come here.
7. You mustn't eat all the cake.
8. She doesn't have to work in the evening.
9. We had to write a poem.
10. You have to bring your books.