Resolved questions
When do you use в or на ?

"В" and "на" - prepositions. "В" is translated as "In, at, into, to, for". In other words it means "inside"
Examples: I live in Moscow - Я живу в Москве
She puts cards in the box - Она кладёт карточки в ящик
He works at school - Он работает в школе
Children play in the garden - Дети играют в саду
She is afraid to go into the forest - Она боится войти в лес
I'm going to Paris - Я собираюсь в Париж
Students go to school every day - Ученики ходят в школу каждый день
He left for Europe - Он уехал в Европу
Professor entered the hall - Профессор вошёл в зал
Young guys had entered the community - Молодые ребята вступили в сообщество
"На" is translated as "on, upon". In other words it means "on the surface"
Examples: Books are on the table - Книги на столе
I hang this picture on the wall - Я повешу картину на стену
She lives in Cyprus (simply, she lives on te surface of the island, planet,earth) - Она живёт на Кипре
Planes fly to the south - Самолёты летят на юг
It's noisy in the street - Шумно на улице
I live in Moscow in Varvarka street - Я живу в Москве на улице Варварка
"На" is also translated as "at":
He is at work - Он на работе (just a fact)
He went to work (with his head) - Он погрузился в работу (с головой). Simply, he went inside his work.
She met a friend at a concert - она встретила друга на концерте. In other words "during the events"
She is going to a concert - она собирается на концерт. Just a fact.
It was fun at the wedding - Было весело на свадьбе ("during the events")