The nominative case is the dictionary form for Russian nouns. The nominative case is the one used as subject of a sentence.
Москва очень красивый город – Moscow is a very beautiful city.
In this sentence the word Moscow is the subject of the sentence and it is in nominative case.
Luckily, in nominative case you will have to remember a few rules to form a plural noun, because it’s the only time when its ending is going to change (check how to form endings of plural nouns in “plural nouns” lesson).
The usage of nominative case:
1. When you talk about a person or a thing performing an action or being in a state, it answers the question кто? что? – Who? What?
Девочка бежит в школу – The girl is running to school
Магазин открыт – The shop is open
2. To define a person or thing or refer them to a certain group of people or things.
Катя студентка – Katia is a student
Яблоко – это фрукт – apple is a fruit
Лондон – это столица Великобритании – London is the capital of Great Britain
3. To indicate a person or a thing towards the action is directed.
Суп был приготовлен мамой – soup was cooked by Mom
Собака не помыта – the dog is not washed.