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Deduction (deciding that something is probable): should, ought to, etc

Present or future​

Should can be used to express the deduction or conclusion that something is probable now or in the future: it is expected, normal or logical.

  • I’ve bought three loaves – that should be enough.
  • I’ll phone Ruth – she should be home by now.
  • ‘We’re spending the winter in Miami.’ ‘That should be nice.’

Ought to (see here) is used in a similar way, but is much less common.

  • Henry ought to be here soon – he left home at six.
  • The weather ought to improve after the weekend.

Past: should have / ought to have …​

We do not normally use should + infinitive to talk about the past.

However, we can use should have / ought to have + past participle to make guesses or draw conclusions about things which are not certain to have happened.

  • James should have got back home yesterday. Has anybody seen him? (not James should get back home yesterday …)

We can also use this structure to talk about actions which we expect to have been completed by now or at a future time.

  • Ten o’clock. She should have arrived at her office by now. I’ll try calling her.
  • We ought to have finished painting the house by the end of next week.

be supposed to​

We can use be supposed to to say that something is or was normal or expected.

  • Cats are supposed to be afraid of dogs, but ours isn’t.
  • Bill was supposed to get back home yesterday.

Note the pronunciation: /səˈpəʊst tə/, not /səˈpəʊzd tə/.

note

For be supposed to to express obligation, (see here).

note

For more about structures with ought, (see here).