How do you define the word "should"? I know what it means, of course, & I looked up what the dictionary says.

Question

But it's hard to define the word "should" without using the word itself.

Answer 1

In sentences like "Should I go or stay?" it means "ought".

In conditional sentences like "If I were to go, I should be sad.", the meaning is like "shall in theory" ~ a hypothetical future reference.

"If I had gone, I should have been sad." is conveying "shall have in theory".

Answer 2

Should, it means there is something that needs to be done, or would be in your best interest to do. Most of the time it conveys that you don't want to do the thing you need to do.

Answer 3

Must.
Ought to.
Supposed to.