You should try this - it's delicious! You should try this - it's delicious!
Should is a modal auxiliary verb which has different uses.
=====Construction=====
affirmative: subject + 'should' + bare infinitive I should go.
negative: subject + 'should' + 'not' + bare infinitive I shouldn't go.
interrogative: 'should' + subject + bare infinitive Should I go?
Note that like other modal auxiliaries, should has no infinitive and we use the same form with 'I', 'you', 'he', 'she', 'it', 'we' and 'they'. 'Do' is not used to form negative or interrogative sentences.
| Use | Examples |
|
Giving and asking for advice |
You should go to the doctor's - you've been coughing for a week. You shouldn't work at weekends - you need a break. Should I phone or email him? |
|
To talk about the best or right thing to do (in one's opinion)
|
Banks should tell customers in advance about charges. We should discuss this with the client before making a decision. Should we have the meeting in the boardroom or your office?
|
|
Deduction |
That should be the postman at the door; he's usually here about this time. |
|
Expectation - this is similar to 'will' for predictions but less certain |
She should do well in her new job; she has all the right experience. It shouldn't rain this afternoon so I won't take an umbrella. |
|
The past tense of 'shall' [See also reported speech.] |
She wondered if she should phone him or send an email.
|
|
As an alternative to 'would' when (i) giving an opinion (ii) in second conditional sentences
|
I should think that's a good restaurant - let's come back later. If I won the lottery, I should give up work and travel round the world. If I (should) see him, I'll give him your message. Should I see him, I'll give him your message. |
==========
These are all used in the same way but with different levels of certainty.
For example, someone hears a knock on the door at the time that the postman usually arrives and says:
That should be the postman. [It's probably the postman but it could be someone else.]
That must be the postman. [I'm certain it's the postman but there's a possibility that it isn't.]
That'll be the postman. [I have no doubt at all that it's the postman.]
=====Had better=====
'Had better' can also be used to talk about the best or right thing to do and is a little stronger than should.
It's late now - I think we'd better postpone the meeting until tomorrow.
affirmative: subject + 'had better' + bare infinitive We'd better go.
negative: subject + 'had better' + 'not' bare infinitive We'd better not go.
interrogative: had' + subject 'better' + bare infinitive Had we better go?
Note that
1. We use it to speak about specific rather than general situations.
You have an exam tomorrow so you'd better/should go to bed early tonight.
When you are taking an exam, you should go to bed early the night before.
2. 'Had better' is preferred for a warning or when there is some urgency.
"I'm going home now." "You'd better not. There's a lot more work to do!"
Oh, no! It's starting to rain. We'd better run!
3. In the interrogative, should is much more common.
=====Ought to=====
Ought to has the same function as a modal auxiliary verb and has different uses.
Construction
affirmative: subject + 'ought to' + bare infinitive You ought to go.
negative: subject + 'ought' + 'not' + 'to' + bare infinitive You oughtn't to go.
interrogative: 1. 'did' + subject + 'ought to' + bare infinitive Did you ought to go?
2. 'ought' + subject + 'to' + bare infinitive Ought you to go? [More formal]
Note that we use the same form with 'I', 'you', 'he', 'she', 'it', 'we' and 'they'.
Ought to has the same meaning as should for advice and talking about expectations or the right thing to do.
Notes
1. Ought to is more common in conversation than in writing but, generally, is used less than should.
2. Should is preferred when we talk about what an outside authority believes is the right thing:
The instructions say that Part A should be connected to Part B.
3. In the interrogative, should is much more common. With ought to, we are more likely to add the verb 'to think'.
Do you think we ought to tell him? 
Ought we to tell him?
4. The negative forms are also rarely used. Should is preferred.
5. Ought to can also be followed by the present continuous.
He ought to be wearing protective clothing.
How should I prepare the corn?
The government should reduce spending.
Should I open it now or wait until my birthday?
My bus leaves at six so I should be there by seven.
The final contract should be ready for you to sign in two days.
He asked me if we should meet at the station or the airport.
We're running out of food - you'd better go and get some more.
Where are my keys? They ought to be in my bag but I can't find them.
You ought to look for another job - you'll never get promoted here.