In British English, the Simple conjugation with the auxiliary should is often used in subordinate clauses stating conditions. This construction is usually used to refer to events that may occur by chance.
e.g. If I should see him, I will tell him what I think.
Should is also used with the meaning ought to. This is the most common use of should in American English.
e.g. You should take an umbrella with you, in case it starts to rain.
I should answer his letter as soon as possible.
Ought is said to be a defective verb, since it has no infinitive, or present or past participle. It does not modify, but has the same form, regardless of the subject. Ought can be used only in combination with other verbs. Unlike the modal auxiliaries, which are followed by the bare infinitive, ought is followed by the infinitive of whatever verb it accompanies.
In each of the following examples, ought is underlined, and the infinitive which follows it is printed in bold type.
e.g. You ought to take an umbrella with you.
He ought to stop smoking.
They ought to drive more carefully.