"She is in the room."
Translation:वह कमरे में है।
Because it is followed by the postposition में, nouns change into the oblique form when followed by a postposition, changing कमरा (masculine singular nouns) into कमरे में, कमरे (masculine plural nouns) into कमरों में, feminine singular nouns like किताब stay the same, feminine plural nouns like किताबें become किताबों में. (There are many postpositions, just using में as an example)
Is there a reason why this is not उस कमरे में है? Wouldn’t the post position में change the वह to उस?
वह changes to उस when the noun is followed by a postposition. Since it's कमरे that's followed by a postposition, that's the word that becomes oblique (कमरा becoming कमरे,) no change happens to वह. उस कमरे में = in the/that room, so उस कमरे में है = it's in the room (है meaning "it is" here.) The room is in her = कमरा उसमें है, because "her" is followed by a postposition, वह becomes उस, and there is no change to कमरा.