Page 543 - Practical English Usage 3ed - Michael Swan, Oxford

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should and ought
Should and ought (see 403) are very similar, and can often replace each other. They ought to be more sensible, shouldn't they?
They are both used to talk about obligation and duty, to give advice, and to say what we think it is right for people to do or have done. Should is much more frequent than ought.
You should / ought to see 'Daughter ofthe Moon' - it's a greatfilm.
You should / ought to have seen his face! Should and ought are not used in polite requests.
Couldyoumoveyourheadabit?Ican'tsee.(NOT ¥Btlsh6~ltl/6~ghtttJ17t6fJe j'6~' head ti bit . ..)
Should and ought are both also used to talk about logical probability. I've bought three loaves - that should / ought to be enough.
That should / ought to be Janet coming upstairs now.
Ought, unlike should, is followed by a to-infinitive. Must is stronger than should/ought
Must has similar meanings to should and ought, but is stronger or more definite. It expresses great confidence that something will happen, or that something is true; should and ought express less confidence. Compare:
- The doctor said I must give up smoking.
(an order which is likely to be obeyed)
You really ought to give up smoking.
(a piece of advice which mayor may not be followed)
- Rob must be at home by now. (= I'm sure he is at home.)
Rob should be at home by now. (= I think he is probably at home.) Should can be used instead of must to make instructions sound more polite.
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This form should be filled in in ink. Applications should be sent by 31 January.
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should (3): should, ought and must 520
It was going to be a long day. I was supposed to clean up all the stables, and then start on the garden. (NOT ••• : s/ttJf;tltl elettn ~" ...)
She was supposed to be in her office, but she wasn't. (NOT She .~ltl be ill her 6fjiee, b~t she wmn't.)
4 should have . ..
Should have + past participle can be used to talk about past events which did not happen, or which mayor may not have happened.
I should have phoned Ed this morning, but I forgot.
Ten o'clock: she should have arrived in the office by now. You shouldn't have called him a fool - it really upset him.
For should after in case, see 271.2.
For should after so that and in order that. see 543.
For How should . •.? and Why should . ..1. see 482.2.
For special uses of should in other subordinate clauses. see 521.
520 should (3): should, ought and must
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