'Should' is a modal verb. Modal verbs are used to give additional information about the main verb. 'Should' is used to talk about assumptions and to give advice. It is the past tense of modal verb 'shall.' For example:

'Ought to' is a semi-modal verb. Semi-modal verbs sometimes act like modal verbs and sometimes act like the main verb. For example:

We use 'should' and 'ought to' to talk about assumptions. The word assumption is derived from 'assume.' Assumptions are statements without any firm proof. For example:

We can use 'should' and 'ought to' to talk about obligations. In other words, we are talking about actions and events that their fulfillment is a duty. 'Ought to' gives a stronger sense of duty than 'should.' Take a look at these examples:

We use 'should' and 'ought to' to talk about things that we expect to happen. For instance:

We can create negative and interrogative forms with 'should' and 'ought to.'
To create negative sentences, we simply add 'not' to our structure as illustrated below:

Here are some examples:

To create questions with 'should,' we simply invert it. Watch:

To make questions with 'ought to,' we follow the pattern shown below:

Here are some examples for clarification:

We use 'should' and 'ought to' with the second type of conditionals that talks about hypothetical situations. These situations are imaginary with a low chance of occurrence. For example:

'Should' is commonly used in all types of texts while 'ought to' is extremely uncommon. 'Should' usually replaces 'ought to.'

'Should' is less formal than 'ought to.' 'Ought to' is used in formal texts while 'should' can be used in both formal and informal contexts.