The modal verb ought to
This modal verb has only one form which is used with reference to the present or future. In indirect speech it remains unchanged.
Ought to has the following meanings:
Obligation implying advisability and desirability.
She ought to warm him. He oughtn’t to mention it to anybody.
In this meaning ought to is used in all kinds of sentences and is followed by the simple infinitive.
With reference to the present ought to is used with the continuous infinitive: At your age you ought to be earning your living.
If the verb does not admit of the continuous form the simple infinitive is used: You ought to feel some respect for your elders. In combination with the perfect infinitive ought in the affirmative form shows that a desirable action was not fulfilled: You ought to have chosen a more suitable time for telling him the news. In the negative form ought to in combination with the perfect infinitive shows that an undesirable action was fulfilled: I’m sorry, I ought not to have said it. You oughtn’t to have married her. It was a great mistake.
Supposition implying strong probability:
The new sanatorium ought to be very comfortable.
The use of ought to in this meaning is not very common. This meaning is usually replaced by must.
The set phrase:
You ought to know it – means you are supposed to know it.
Shall / should
Historically shall and should were two forms of the same verb: shall was the Present tense Indicative mood, should was the form of the Subjunctive mood. But later they came to express different meanings, and in present-day English they are treated as two different verbs.
Shall
In modern English the modal meaning of obligation is always combined in shall with the function of an auxiliary verb of the future tense.
Shall is still used to express obligation with the second and third persons, but at present it is not common in this meaning, it is used in formal, archaic style and is mainly found in subordinate clauses
It was decided that the proposal shall not be opposed.
At present the modal verb shall is found with the second and third persons in sentences expressing promise, threat or warning. It is used in affricative and negative sentences and is combined with the simple infinitive:
You shall have my answer tomorrow (promise)
He shall not come off with a whole skin this time.(threat)
‘You shall stay just where you are!’ his mother cried angrily.
With the first person shall is used in interrogative sentences asking for instructions: Shall I get you some fresh coffee. Who shall answer the telephone? Shall I answer in English? Shall I open the window?
Should
In modern English the modal verb should is used with reference to the present or future, and it remains unchanged in indirect speech. Should has the following meanings:
Obligation combined with advisability:
It’s late. You should go to bed.
You shouldn’t miss this opportunity.
In combination with the perfect infinitive should in the affirmative form shows that a desirable action was not fulfilled: He looks very ill. He should have stayed at home. You should have told me about it yourself.
In the negative form should in combination with the perfect infinitive shows that an undesirable action was fulfilled: You shouldn’t have done as you did.
Should occurs in object clauses and in adverbial clauses of condition, purpose and concession: I suggest that you should stay here. It is important that people should know about it. If he should drop in, give him my message.
The use of the so-called “emotional should” is found in the following cases: Why should I do it? Why shouldn’t you invite him?