Here is something about cases.

when we talk of case, we mean the way in which a word changes its form or ending according to its position in the sentence. in english, for example, when the pronouns ‘i’, ‘he’, ‘she’, etc. are the object (i.e. on the receiving end) of a verb or are preceded by a preposition like ‘in’, ‘to’, ‘on’, they change their form to ‘me, him, he’, etc.: ‘i see him’; ‘to me’; ‘on him’.
similarly in urdu, when میں is followed by a postposition such as سے, پر, کو it changes its form to مجھ ‘me’ [...] میں is the direct case and مجھ is the oblique case.