The particle ni
Ni (に) is a particle that has several different functions in Japanese. One main function is to tell where a thing or person is located.
Smith is in Tokyo = sumisu-san wa toukyou ni imasu
(I am) at girlfriends house = kanojo no ie ni imasu
(I am) at room = heya ni iru (部屋にいる)
Key is in room = kagi wa heya ni aru (かぎは部屋にある)
Prepositions (in, on, under) are used in Japanese. The sentence pattern they are found in are different from English, but they are easy to remember. First we introduce you to naka (中) which means inside or the middle. Below is pictures using a cat to show examples.
The pattern (something) no naka means, something's inside. This same pattern is used for other prepositions as well.
The word ue (上) actually means up but when its used as a preposition it means on top of or above.
Shita (下) means down. This preposition is used to mean something is located below or under something.
Mae (前) means before, it is used to show that something is situated in front of or before something.
Yuki is before the door = yuki-san wa doa no mae ni imasu
Before the car, there is a cat = kuruma no mae ni, neko ga iru
The Japanese preposition that is used for outside is soto (外).
Outside the house = ie no soto (家の外)
I am outside = watashi wa soto ni iru (私は外にいる)
I am outside the house = watashi wa ie no soto ni imasu