Pronouns, possessives and there is

subject and object: The subject does the action and stands before the verb; the object receives it and stands after. English has a different pronoun for each job — I see him, he sees me — and the position alone tells you which one you need.

possessive: The possessive says who owns the thing, and it agrees with the OWNER, never with the thing owned: his sister, her brother, their house. It does not change for gender or number of what follows — my book and my books are both my.

there is / there are: there is and there are announce that something exists, or is present somewhere. The there is not a place: it is an empty word that holds the subject position open, because English refuses to start a sentence without one.