oxford_guide_to_english_grammar

12 MODAL VERBS PAGE 118
4 Necessity not to do something
a
b
c
We use mustn't to tell someone not to do something.
You mustn't forget your railcard. We mustn't lose this game.
The meaning is the same as Don't forget your railcard. The speaker feels the
necessity. Compare You must remember your railcard.
Mustn't has a different meaning from needn't/don't have to. Compare these
sentences.
I needn't run. I've got plenty of time.
I mustn't run. I've got a weak heart.
We can use mustn't or may not to forbid something.
Students must not/may not use dictionaries in the examination.
Here the speaker or writer is the authority, the person who feels the necessity to
stop the use of dictionaries. But if we are talking about rules made by other people,
we use can't or be allowed to. • 94(3)
We can't use/We aren't allowed to use dictionaries in the exam.
93 Obligation and advice: should, ought to etc
1 Should and ought to
a
b
We use should and ought to for obligation and advice, to say what is the right thing
or the best thing to do.
They should build/ought to build more hospitals.
People shouldn't leave/oughtn't to leave litter all over the place.
You should go I ought to go to York. It's an interesting place.
I shouldn't leave/oughtn't to leave things until the last moment.
Who should we invite?/ Who ought we to invite 1 .
Should and ought to are not as strong as must.
You should tour in a group. (It's a good idea to.)
You must tour in a group. (It's essential.)
But in formal rules should is sometimes a more polite and less emphatic
alternative to must.
Passengers should check in at least one hour before departure time.
We can use the continuous or perfect after should and ought to.
I should be doing some work really.
You should have planted these potatoes last month.
After all the help Guy has had, he ought to have thanked you.
The perfect here means that the right action did not happen. Compare had to,
where the action really happened.
I ought to have left a tip.
(Leaving a tip was the right thing to do, but I didn't leave one.)
I had to leave a tip.
(It was necessary to leave a tip, so I did leave one.)