Prepositions В and НА

The main difficulty in the usage of Russian prepositions is that there are lots of exceptions. However, at first we should understand what preposition to use with one or another word.

If we are talking about material objects such as a table, cupboard, fridge, box, dresser, the preposition "На" [na] means the location of any object on the surface (on top) and "В"[v] - inside of an object.
For example:
Положить одежду в комод - To put clothes in a dresser.
Положить одежду на комод - To put clothes on a dresser.

Положить учебник на стол - To put a textbook on a desk.
Положить учебник в стол - To put a textbook in a desk.

We can use both of these prepositions: "В" and "На" even when we are talking about objects which don't have distinct boundaries e.g. grass.

For example:
Мы устроили пикник на траве - We had a picnic on the grass.
В траве спрятался кролик - A rabbit hid in the grass.

So if one object is situated on the surface of another, we use the preposition "На" [na] - on.

Attention! This surface is open and not limited from above. You should remember this because that kind of thinking helps to understand logiс and culture of the language. That also contributes to the formation of linguistic feeling.

For example:
на улице - in the street
на мосту - on a bridge
на берегу - on shore
на остановке - at the station
на площади - on the square
на балконе - on the balcony
на стадионе - at the stadium
на рынке - on the market
на стуле - on a chair
на диване - on the couch
на столе - on the table
на тарелке - on a plate
на горе - on the mountain
на острове - on the island

We also use the preposition "На", when we are talking about cardinal points:
На юге - in the South, на севере - in the North, на западе - in the West, на востоке - in the East.

But there are several exceptions. Here are some of them:
We use the preposition "В" [v] (in), when we are talking about reservoir (meaning being in the water):
В реке плавает рыба - There is fish in the water.
Мы купаемся в озере - We are swimming in the lake.

The preposition "В" is also used with the word "кресло" [kr'esla] (armchair), if we are talking about a person sitting in an armchair. And with the word "рука" [ruka] (hand).
Люблю сидеть в удобном кресле - I like sitting in a cozy armchair.
Малышей учат держать ложку в руке - People teach children to hold the spoon in hand.

We use the preposition "В", when we are talking about the object which is situated in a radius of another object. It means the second object is something enclosed like a box. Or having distinct boundaries.

For example:
в квартире - in a flat
в доме - in a house
в стране - in a country
в парке - in the park
в саду - in the garden

Exceptions:
на Родине - in the homeland
на вокзале - at the station
на складе - in storage
на фабрике - at the factory
на заводе - at the plant
на этаже - on the floor

If we are talking not about material objects but about events, most often we use the preposition "На".
For example:
на уроке - in class
на опере - at the opera
на выставке - at the exhibition

We can also say "в опере" (in the opera) but then the word "опера" would mean not event but a building with walls and roof. So it would mean "in the opera house"

Exceptions:
в путешествии - on the journey
в поездке - on the trip
в дороге - on the road
в отпуске - on vacation
в командировке - on a business trip
в кино - at the movies

We can't mention in this article all the exceptions connected with the usage of prepositions, that's why you can see other ones which you should just memorize.