^^^ Same as he said. I think the only way I finally nailed it down was when I did the Michel (whatever his last name) method. "De" is used when you are saying you did something somewhere. - You studied "at" home. うちでべんきょうをしました。 - You drank coffee "at" starbucks.　スタバでコーヒーをのみました。 If you want to say something exists in a certain place, you use NI. Heya ni neko ga imasu. へやにねこがいます。 There is a cat in the room. So you'll pretty much use, "Ni" with arimasu or imasu. Generally speaking. Or with your action verbs...Ikimasu, kimasu...blah blah blah...

To indicate the place where the action happens, you must use "de". So the sentence should be: "Ashita uchi de benkyou shimasu"