GMAT Verbal Section | Re: The antecedent for pronoun "IT"

"It" can have an antecedent and sometimes it works as a dummy subject(forgot the exact terms used someone mentioned in one post). I always get confused where it should have an antecedent and where it should not.
For instance--

she is going out, and it is raining today (no antecedent)

She is working on a case, and it is not getting resolved.(it refers to case)

Above were very simple examples, so anyone can easily identify in which sentence "it" should have antecedent; however,