Russian words take different forms depending on their role in the sentence. These forms are called cases. A few forms may look the same (cf. "frequent rains" vs. "It rains often").
These forms have names (mostly calques from Latin) that describe some "prototypal" use of such case: Nominative, Accusative, Genitive, Prepositional, Dative and Instrumental. For you, these are just tags: the use is what defines a case.
As of now, you know the NOMINATIVE case: the dictionary form of a word. This form acts as the grammatical subject of the sentence, the "doer". It is also used for both nouns in "A is B" structure:
You also know a few Genitive forms (у меня) but that's it. For now, we will tackle something easier.
When we talk about things being somewhere, we typically use в(in) or на (on) with the Prepositional form of the noun. It doesn't work when you mean motion to that place!
The Prepositional case (a.k.a. Locative) is the only case that is never used on its own without a preposition, even though only four or five prepositions ever use it:
Unlike English (“at/in school”), in Russian each "place" is associated with just one preposition. The rough overall rule is: use “в”(in, at) when talking about buildings and places with certain boundaries and use “на” (on, at) when talking about open spaces or events:
When you mean physically being inside/on top of some object, there is little ambiguity. "Places", unfortunately, require memorization.
Here is the rule that covers most nouns:
Use “у + Genitive” when talking about being at some person’s place: Да, я у дру́га = Yeah, I am at my friend’s place.
The room with a toilet is туале́т. In this course, we stick to the North American "bathroom", even though a room with a bath is, technically ва́нная (it has ва́нна, "a bath"). Still, in Russian you would not ask for a "bath-room" unless you really mean it.
We’ll deal with that later. But the pattern is consistent. When you are somewhere, going to that place and going away from that place, use the following triplets:
| AT | TO | FROM |
|---|---|---|
| в + Prep | в + Acc | из + Gen |
| на + Prep | на + Acc | с + Gen |
| у + Gen | к + Dat | от + Gen |
For example, if the place is used with на, the correct prepositions for the three uses are на–на–с.