'Should' is a modal verb. Modal verbs give additional information about the main verb. We use it to talk about possibilities, make suggestions, give advice, predict the future, etc. 'Should' is the past tense of the modal verb 'shall.' For instance:

'Supposed to' is a phrase that has a similar function to modal verbs. That is why it is called 'modal verb phrase.' 'Supposed to' is used to talk about duty, and intended events and actions. For instance:

We use 'should' and 'supposed to' to talk about obligations and duties. Duties and obligations are actions that are legally and morally required to be fulfilled. 'Should' represents a weaker obligation. For instance:

We use 'should' and 'supposed to' to talk about actions or events that we expect them to happen. These events have been preplanned and programmed. For instance:

We use 'should' to ask for or give advice. When we are giving advice, we are expressing what we thing is the right thing to do. When we are asking for advice, we are asking for the right ways to do something. Have a look:

'Should' and 'supposed to' can create negative sentences:

To create negative sentences with 'should,' we simply add 'not' after 'should' as illustrated below:

Here are some examples for clarification:

To make a sentence that contains 'supposed to' negative, we follow the pattern shown below:

Watch:

'Should' and 'supposed to' can create interrogative sentences:

To create questions, we invert 'should' as illustrated in the examples below:

Creating questions with 'supposed to' is a little tricky. Unlike modal verbs, we do not change the position of 'supposed to.' We invert the 'to be' verb as shown in the pattern below:

For example: