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Well, I have studied sentences in which should acts for bringing possibility. In the below two sentences, I am confused about its use in an usual form.
We have to be prepared should they get here.
Does "should" brings possibility or a more likely outcome when used in place of "if"?
Under no circumstances should you promise to help him if you can't deliver the goods.
In this case, how is "should" acting like?
In your first example sentence:
'We have to be prepared should they get here.'
A sentence with a similar meaning would be:
'We have to be prepared if and/or when they get here.'
Replacing the word 'should' with 'if' by itself alters the meaning of the sentence to imply that their arrival is less sure. This is because 'should' normally implies something that 'should' or is more likely to happen, while 'if' is neutral about what might happen.
In your second example:
'Under no circumstances should you promise to help him if you can't deliver the goods.'
The speaker in this sentence is advising the listener about what to do. 'Should' in this case has the meaning of what is the correct action, so if 'you can't deliver the goods' (i.e. if you can't help him) your correct course of action is to not 'promise to help him'.
