Page 225 - Basic English Usage

227 295

Deduction
We can use should to say that something is possible (because it is
logical or normal).
Henry should be here soon — he left home at six.
‘We're spending the winter in Miami.’ ‘That should be nice.’
should have ...
We can use | should + perfect infinitive ; to talk about the past. This
structure is used to talk about things which did not happen, or which
may or may not have happened (see 202.3).

should + have + past participle |

| should have phoned Ed this morning, but! forgot.
Ten o'clock: she should have arrived in her office by now.

Conditional
Should/wouid is a conditional auxiliary (see 88).
| should/would be very happy if | had nothing to do.
For should after in case, see 172. For should in that-clauses, see 332.1.
For should and would, see 296.

295 should, ought and must

Should and ought are very similar. 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. (See 294 and 232.)
You ought to/should see ‘Daughter of the Moon’ it's a great film.
—
There is sometimes a small difference. We use should or ought when we
are talking about our own feelings, but we prefer ought when we are
talking about ‘outside’ rules, laws, moral duties etc.
Everybody ought to give five per cent of their income to the Third
World.

Must is much stronger than should and ought. For example, we can give
advice with should and ought, we can give orders with must. Compare:
You ought to give up smoking. (= It's a good idea.)
The doctor said | must give up smoking. (= He told me to.)
We can use should and ought to say that something is probable; we can
use must to say that it is certain. Compare:
Henry ought to be at home now. (= There is a good reason to think
he’s at home.) ‘
Henry must be at home now. (= There are reasons to be certain that
he’s at home.)