Should was originally the past tense of shall, but it has acquired new shades of meaning in the course of its development.

The modal verbs should and ought are treated together here as there is hardly any difference between them. Very often they are interchangeable.

I ought to have married; yes, I should have married long ago. (Poittsma)

There is, however, a difference in construction. Whereas should is followed by the infinitive without the particle to, ought is always followed by the to-infinitive.

When reference is made to the present or future, the Indefinite Infinitive is used.

In wartime a man should not part with his rifle. (Heym)

It’s murder, and we ought to stop it. (London)

When reference is made to the past the Perfect Infinitive shows that the obligation was not carried out.

“You should have been here last night when they brought back the DP’s to the mines,” said Yates. (Heym).

She ought to have known that the whole subject was too dangerous to discuss at night. (Galsworthy)

I know that I was weak in yielding to my mother’s will. I should not have done so. (London)

She had no nerves; he ought never to have married a woman eighteen years younger than himself. (Galsworthy)

Should and ought are sometimes used with the Continuous Infinitive and the Perfect Continuous Infinitive.

You should be learning your lessons, Jack, and not talking with Mary.

You ought to be helping your mother with your salary and not squandering your money.

He should have been trying to break through the isolation the hospital had set around Thorpe, he should have been doing many things other than walking along the Seine quay. (Heym)

Both should and ought express obligation, something which is advisable, proper, or naturally expected.

#1. Obligation, very often a moral obligation duty. In this meaning ought is more frequently used than should.

Martin’s hand instinctively closed on the piece of gold. In the same instant he knew he oughtn’t to accept. (London).

I promised her if ever the time came when she needed me, to be her friend. Promises of that sort should never be broken. (Meade)

#2. Advisability

In this meaning should is more common than ought, as it always shows some personal interest whereas ought is more matter-of-fact.

You should be more careful. (London)

You ought to have Warmson to sleep in the house. (Galsworthy)

#3. Something which can be naturally expected

It’s the last of the Madeira I had from Mr. Jolyon… it ought to be in prime- condition still. (Galsworthy)

If it’s a story by Wodehouse it should be amusing.