the parliament can't legislate on a subject in the state list except....(a)
if the Rajya Sabha passes any of the subjects of the state list with 2/3 majority of the present and eligible voters of the House, declaring the subjects to have national importance, on that subject parliament acquires the power to legislate i.e. to make laws. This can be implemented for one year. If Rajya Sabha wants, the term of one year can be increased to one more year.
(b) According to the 7th Schedule of the Constitution, parliament makes law. If in any way states are related to this law or any state has implemented laws on the same subject, then the law by the parliament will be implemented.
(c) if in the whole country or in any part of it, emergency is declared, then parliament acquires the power to make laws regarding the subjects mentioned in all the lists including state lists for the whole of the country or a part of it. The same arrangements are implemented when emergency is declared.